Add datasource for generating AWS external IDs for role assumption

Description of the change

Added a new data source for generating external IDs and assume role policies for a spacelift_aws_integration_attachment.
Added our AWS account ID to the spacelift_account data source for help when creating AssumeRole policies.
Added some examples showing how to use the new attachable integration.
Added retries to AWS attachment to work around eventual consistency.

[ ] Bug fix (non-breaking change that fixes an issue)
[x] New feature (non-breaking change that adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] Documentation (non-breaking change that adds documentation)

[x] Lint rules pass locally
[x] The code changed/added as part of this pull request has been covered with tests
[x] All tests related to the changed code pass in development
[x] tfplugindocs has been run to make sure the docs are up to date.

[x] This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
[x] Pull Request is no longer marked as "draft"
[ ] Reviewers have been assigned
[x] Changes have been reviewed by at least one other engineer
[x] The target branch is future unless the change is going directly into production
[x] DO NOT MERGE until https://github.com/spacelift-io/terraform-provider-spacelift/pull/285 is merged.