HCPE-830 - Add TGW attachment resource

[roaks3]

Ticket: https://hashicorp.atlassian.net/browse/HCPE-830

Adds the hcp_aws_transit_gateway_attachment resource and data source. The implementation is largely pulled from the initial implementation: https://github.com/hashicorp/cloud-terraform-provider-hcp-internal/pull/8

The inclusion of aws in the resource name came from some discussion captured here, which might change moving forward.

A simple example of what the resource looks like:

resource "hcp_aws_transit_gateway_attachment" "example" {
  hvn_id                        = hcp_hvn.main.hvn_id
  transit_gateway_attachment_id = "example-tgw-attachment"
  transit_gateway_id            = aws_ec2_transit_gateway.example.id
  resource_share_arn            = aws_ram_resource_share.example.arn
  destination_cidrs             = [aws_vpc.example.cidr_block]
}

To Test

The example included here can be used to test the functionality of this resource. Note that the AWS config cannot be the same account as the HVN.

[roaks3]

The docs/* files included here were generated with go generate, which pulls in the examples and resource/field descriptions.

[bcmdarroch]

Had some questions and a suggestion for the new provider_account_id HVN output, but otherwise nice work! ✨

[bcmdarroch]

Let's save this for a follow-up, but I think this is a good point to introduce the pattern for returning cloud-specific outputs. I'm thinking we'll have an output with the top-level name aws, which contains account_id, similar to this vpc_config object in the the AWS provider's eks_cluster resource. It's a bit of a hack, but it sounds like it's the best we've got while the provider SDK team works out a better solution for input/output objects.

[roaks3]

Yea, I think this would warrant some more discussion, so my preference would be to include that in the general topic around cloud-specific values in our network resources. Definitely agree that this field falls into that category.

[smaant]

A few comments

[smaant]

Great job @roaks3, thank you for handling this!

[nombiezinja]

Awesome work, appreciate you taking this on!