optionally specify additional user agent details

[jaymccon]

This PR enables Terraform configuration authors to optionally specify additional user-agent data that terraform passes on to AWS Cloud Control API's.

This is useful for discovery and compliance purposes, as the user-agent for each API call is available as a CloudTrail event. Teams can use this data to identify workloads running in their account such as 3rd party products, OU's, etc.

In addition to unit tests, functional tests were done by compiling the patched provider locally and running apply with TFLOG=TRACE set to print the raw api calls. Using this tf module:

provider "awscc" {
  region     = "us-east-1"
  user_agent = [
    {
      name    = "my-test-module"
      version = "0.0.1"
      comment = "testing user-agent comment" 
    }
  ]
}

resource "awscc_ec2_vpc" "test-vpc" {
  cidr_block = "10.0.0.0/16"
}

Checking both the Terraform trace output and the resulting event in CloudTrail I was able to confirm that the User-Agent had the custom data in addition to the default Terraform user-agent data:
APN/1.0 HashiCorp/1.0 Terraform/1.0.8 (+https://www.terraform.io) terraform-provider-awscc/dev (+https://registry.terraform.io/providers/hashicorp/awscc) my-test-module/0.0.1 (testing user-agent comment) aws-sdk-go-v2/1.9.1 os/macos lang/go/1.17 md/GOOS/darwin md/GOARCH/amd64 api/cloudcontrol/1.0.0

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• The resources and data sources in this provider are generated from the CloudFormation schema, so they can only support the actions that the underlying schema supports. For this reason submitted bugs should be limited to defects in the generation and runtime code of the provider. Customizing behavior of the resource, or noting a gap in behavior are not valid bugs and should be submitted as enhancements to AWS via the CloudFormation Open Coverage Roadmap.

[gdavison]

Thanks for submitting this PR, @jaymccon. I've updated the User-Agent handling code to append it to the User-Agent string instead of adding it to the APN info. I've also added documentation.