Add ability to lookup tgw vpc attachment id from vpc-id

[JoshiiSinfield]

When using this module in a code repo away from the attachment code, meaning Teerragrunt dependencies aren't available, it'd be useful to allow looking up the vpc attachment id via the data source.

https://registry.terraform.io/providers/hashicorp/aws/latest/docs/data-sources/ec2_transit_gateway_vpc_attachment