Add support for migration and creation-source on the Stack Resource

[aaabdelgany]

Description

This PR adds support for the migration flag thats used when creating Stacks as well as the read only creation-source attribute.

The migration flag in the stack resource is only to be used at the time of creation and it isn't serialized at any further point in an api response for that stack. So this PR has to preserve the migration value using the state file to ensure that terraform doesn't panic because it thinks that it lost state.

The creation-source field is a read-only field that is returned as part of an api response for a stack.

Remember to:

• Update the Change Log
• Update the Documentation

Testing plan

resource "tfe_organization" "foobar" {
  name  = "%s"
  email = "admin@tfe.local"
}

resource "tfe_agent_pool" "foobar" {
  name                  = "agent-pool-test-example"
  organization          = tfe_organization.foobar.name
}

resource "tfe_project" "example" {
	name         = "example"
	organization = tfe_organization.foobar.name
}

resource "tfe_oauth_client" "foobar" {
  organization     = tfe_organization.foobar.name
  api_url          = "https://api.github.com"
  http_url         = "https://github.com"
  oauth_token      = "%s"
  service_provider = "github"
}

resource "tfe_stack" "foobar" {
	name        = "example-stack"
	description = "Just an ordinary stack"
  project_id  = tfe_project.example.id
  agent_pool_id = tfe_agent_pool.foobar.id
	migration = true
	vcs_repo {
    identifier         = "%s"
    oauth_token_id     = tfe_oauth_client.foobar.oauth_token_id
  }
}

External links

Include any links here that might be helpful for people reviewing your PR. If there are none, feel free to delete this section.
hashicorp/go-tfe#1266

Output from acceptance tests

Please run applicable acceptance tests locally and include the output here. See testing.md to learn how to run acceptance tests.

If you are an external contributor, your contribution(s) will first be reviewed before running them against the project's CI pipeline.

TF_ACC=1 TF_LOG_SDK_PROTO=OFF go test $(go list ./... |grep -v 'vendor') -v -run TestAccTFEStackResource_basic -timeout 15m
? github.com/hashicorp/terraform-provider-tfe [no test files]
testing: warning: no tests to run
PASS
ok github.com/hashicorp/terraform-provider-tfe/internal/client (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok github.com/hashicorp/terraform-provider-tfe/internal/logging (cached) [no tests to run]
=== RUN TestAccTFEStackResource_basic
--- PASS: TestAccTFEStackResource_basic (14.21s)
PASS
ok github.com/hashicorp/terraform-provider-tfe/internal/provider 15.054s
? github.com/hashicorp/terraform-provider-tfe/internal/provider/helpers [no test files]
? github.com/hashicorp/terraform-provider-tfe/internal/provider/planmodifiers [no test files]
? github.com/hashicorp/terraform-provider-tfe/internal/provider/validators [no test files]
? github.com/hashicorp/terraform-provider-tfe/version [no test files]

...

Rollback Plan

Changes to Security Controls

[ctrombley]

Looks good!