Adds support for Magic Transit GRE tunnels

[cehrig]

This PR adds a new resource for managing GRE tunnels. The resource layout looks as follows:

resource "cloudflare_gre_tunnel" "example" {
  account_id              = "c4a7362d577a6c3019a474fd6f485821"
  name                    = "GRE_1"
  customer_gre_endpoint   = "203.0.113.1"
  cloudflare_gre_endpoint = "203.0.113.1"
  interface_address       = "192.0.2.0/31"
  description             = "Tunnel for ISP X"
  ttl                     = 64
  mtu                     = 1476
  health_check_enabled    = true
  health_check_target     = "203.0.113.1"
  health_check_type       = "reply"
}

cloudflare/cloudflare-go#798 is required for this to work. The provided acceptance tests will only work when using an account with Magic Transit enabled. They have been tested against a production account:

$ go test $(go list ./...) -v -run TestAccCloudflareGRE
?       github.com/cloudflare/terraform-provider-cloudflare     [no test files]
=== RUN   TestAccCloudflareGRETunnelExists
--- PASS: TestAccCloudflareGRETunnelExists (30.11s)
=== RUN   TestAccCloudflareGRETunnelUpdateDescription
--- PASS: TestAccCloudflareGRETunnelUpdateDescription (46.98s)
=== RUN   TestAccCloudflareGRETunnelUpdateMulti
--- PASS: TestAccCloudflareGRETunnelUpdateMulti (53.26s)
PASS
ok      github.com/cloudflare/terraform-provider-cloudflare/cloudflare  130.362s
?       github.com/cloudflare/terraform-provider-cloudflare/tools/cmd/changelog-check   [no test files]
?       github.com/cloudflare/terraform-provider-cloudflare/version     [no test files]

[github-actions]

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[jacobbednarz]

@cehrig i had to cast MTU and TTL to an int in 9c24b04 as uint{8,16} are incompatible with d.Set (see https://github.com/cloudflare/terraform-provider-cloudflare/runs/5151807097?check_suite_focus=true#step:4:230). would you mind re-running these tests now that i've pulled in the latest cloudflare-go changes and we can get this out?

[cehrig]

Weird, now I am wondering why I did not run into that issue.
Tests are running fine

$ go test $(go list ./...) -v -run TestAccCloudflareGRE
?       github.com/cloudflare/terraform-provider-cloudflare     [no test files]
=== RUN   TestAccCloudflareGRETunnelExists
--- PASS: TestAccCloudflareGRETunnelExists (32.14s)
=== RUN   TestAccCloudflareGRETunnelUpdateDescription
--- PASS: TestAccCloudflareGRETunnelUpdateDescription (50.93s)
=== RUN   TestAccCloudflareGRETunnelUpdateMulti
--- PASS: TestAccCloudflareGRETunnelUpdateMulti (54.20s)
PASS
ok      github.com/cloudflare/terraform-provider-cloudflare/cloudflare  137.278s
?       github.com/cloudflare/terraform-provider-cloudflare/tools/cmd/changelog-check   [no test files]
?       github.com/cloudflare/terraform-provider-cloudflare/version     [no test files]

[jacobbednarz]

the type incompatibility was picked up by tfproviderlintx which runs on pull requests but requires explicit running locally via make terraform-provider-lint. there is make lint that runs everything but may be a little noisy since tools like golangci-lint only checks current changes, not everything.