Draft: Add Public IPv4 Pools data source

[theonlymalwhere]

Description

Presently, there are some gaps in coverage relating to IPv4 Pools. #17425 raises the following user story (among others):

As an IaC engineer, I want to be able to get a list of IPv4 address pools we've added to a specific AWS account via BYOIP so that we can utilise the ranges programatically in the remaining infrastructure.

This gap impacted my org as well (and I have seen several random users on various sites requesting this particular feature), which is why I am submitting this PR, in part-fulfillment of the needs of #17425.

This PR therefore adds the following items:

1. A new data source (located in the VPC section of the docs, but in the ec2 service directory) called aws_vpc_public_ipv4_pools
2. Tests and docs for the foregoing data source.

A substantial question in my mind when I was writing this code was: does it go too far for a data source? Most data sources that can read & provide data on a range of resources only include, as output:

• The IDs of those resources
• An ID for the data source itself (generally, just a meta call back to the region).

This is obviously going a step further in providing structured output according to the DescribePublicIpv4Pools API method. I would appreciate any feedback on whether or not that is, categorically, too much output to include in a Data Source. But on the other hand, users (including myself) have need of a method that simply takes one or more IPv4 pool IDs and returns information about those pools. This is how I chose to implement that, but I am open to requests for changes.

A further question I have for reviewers would be: Have I correctly inferred the error type for my Finder in internal/service/ec2/find.go:1234? I could not find much in the way of documentation about writing Finders in general.

Finally: I am working on acceptance testing, and this has Draft status until I get tests passing. However, due to the nature of this particular resource, it is quite difficult to run Acceptance Tests; we would need to be able to provision a BYOIP pool of public IPs (and this is kinda cost-prohibitive, as well as requiring a manual step to sign and attach a CIDR Authorization Context to the IPAM pool in order to create an IPAM pool CIDR. Any input on how to test this in the existing framework...I am open to your thoughts.

I should note here that all existing tests for BYOIP AND all IPAM variations simply skip any kind of AccTest for public IPv4 addresses, for obvious reasons. There's really nothing I can see in the existing framework to to pattern this approach off of, and I'm struggling to think how we would do this--apart from, e.g., spoofing the existence of public IPv4 pools in the tfstate.

Relations

Relates #17425

Output from Acceptance Testing

$ make testacc TESTS=TestAccVPCPublicIpv4PoolsDataSource PKG=ec2
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/ec2/... -v -count 1 -parallel 20 -run='TestAccVPCPublicIpv4PoolsDataSource'  -timeout 180m
=== RUN   TestAccVPCPublicIpv4PoolsDataSource_filter
    public_ipv4_pools_data_source_test.go:16: Step 1/1 error: Error running apply: exit status 1

        Error: creating IPAM Pool (ipam-pool-0bce7e0554f54c98c) CIDR: InvalidParameterCombination: The request must contain a CIDR Authorization Context for ipam-pool-0bce7e0554f54c98c. The specified pool belongs to a public scope where the specified CIDR has not previously been provisioned.
        	status code: 400, request id: 8805a3a8-9ed2-4ef6-a9df-ed6ffff12639

          with aws_vpc_ipam_pool_cidr.test_cidr_1,
          on terraform_plugin_test.tf line 29, in resource "aws_vpc_ipam_pool_cidr" "test_cidr_1":
          29: resource "aws_vpc_ipam_pool_cidr" "test_cidr_1" {


        Error: creating IPAM Pool (ipam-pool-0bce7e0554f54c98c) CIDR: InvalidParameterCombination: The request must contain a CIDR Authorization Context for ipam-pool-0bce7e0554f54c98c. The specified pool belongs to a public scope where the specified CIDR has not previously been provisioned.
        	status code: 400, request id: bf9f7023-9429-484a-a26d-f0630f4ef76e

          with aws_vpc_ipam_pool_cidr.test_cidr_2,
          on terraform_plugin_test.tf line 34, in resource "aws_vpc_ipam_pool_cidr" "test_cidr_2":
          34: resource "aws_vpc_ipam_pool_cidr" "test_cidr_2" {


        Error: creating IPAM Pool (ipam-pool-0c4c11e0a0f45f1be) CIDR: InvalidParameterCombination: The request must contain a CIDR Authorization Context for ipam-pool-0c4c11e0a0f45f1be. The specified pool belongs to a public scope where the specified CIDR has not previously been provisioned.
        	status code: 400, request id: 1de43236-1e53-43c5-bf9c-39c0ea701528

          with aws_vpc_ipam_pool_cidr.test_cidr_3,
          on terraform_plugin_test.tf line 39, in resource "aws_vpc_ipam_pool_cidr" "test_cidr_3":
          39: resource "aws_vpc_ipam_pool_cidr" "test_cidr_3" {


        Error: creating IPAM Pool (ipam-pool-0c4c11e0a0f45f1be) CIDR: InvalidParameterCombination: The request must contain a CIDR Authorization Context for ipam-pool-0c4c11e0a0f45f1be. The specified pool belongs to a public scope where the specified CIDR has not previously been provisioned.
        	status code: 400, request id: 6a99e112-ee23-46e4-9d4b-9a47df84b42f

          with aws_vpc_ipam_pool_cidr.test_cidr_4,
          on terraform_plugin_test.tf line 44, in resource "aws_vpc_ipam_pool_cidr" "test_cidr_4":
          44: resource "aws_vpc_ipam_pool_cidr" "test_cidr_4" {

--- FAIL: TestAccVPCPublicIpv4PoolsDataSource_filter (73.49s)

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[theonlymalwhere]

Sorry, created a small gitastrophe trying to rebase.

[theonlymalwhere]

Honestly not worth it to fix this branch, I'm just going to kill it & start a new one. Sorry for being a doofus with git!

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