Stacks support (Beta)

[brandonc]

Description

Adds Beta support for Stacks

Testing plan

Tests are subject to Beta skips. See output below.

Output from tests

Including output from tests may require access to a TFE instance. Ignore this section if you have no environment to test against.

$ OAUTH_CLIENT_GITHUB_TOKEN=ghp_xxx go test ./... -run "TestStack" -v
?   	github.com/hashicorp/go-tfe/examples/backing_data	[no test files]
?   	github.com/hashicorp/go-tfe/examples/configuration_versions	[no test files]
?   	github.com/hashicorp/go-tfe/examples/organizations	[no test files]
?   	github.com/hashicorp/go-tfe/examples/registry_modules	[no test files]
?   	github.com/hashicorp/go-tfe/examples/run_errors	[no test files]
?   	github.com/hashicorp/go-tfe/examples/state_versions	[no test files]
?   	github.com/hashicorp/go-tfe/examples/users	[no test files]
?   	github.com/hashicorp/go-tfe/examples/workspaces	[no test files]
?   	github.com/hashicorp/go-tfe/mocks	[no test files]
=== RUN   TestStackCreateAndList
=== RUN   TestStackCreateAndList/List_without_options
=== PAUSE TestStackCreateAndList/List_without_options
=== RUN   TestStackCreateAndList/List_with_project_filter
=== PAUSE TestStackCreateAndList/List_with_project_filter
=== RUN   TestStackCreateAndList/List_with_name_filter
=== PAUSE TestStackCreateAndList/List_with_name_filter
=== RUN   TestStackCreateAndList/List_with_sort_options
=== PAUSE TestStackCreateAndList/List_with_sort_options
=== RUN   TestStackCreateAndList/List_with_pagination
=== PAUSE TestStackCreateAndList/List_with_pagination
=== CONT  TestStackCreateAndList/List_without_options
=== CONT  TestStackCreateAndList/List_with_sort_options
=== CONT  TestStackCreateAndList/List_with_name_filter
=== CONT  TestStackCreateAndList/List_with_project_filter
=== CONT  TestStackCreateAndList/List_with_pagination
--- PASS: TestStackCreateAndList (9.00s)
    --- PASS: TestStackCreateAndList/List_with_project_filter (2.04s)
    --- PASS: TestStackCreateAndList/List_with_name_filter (2.32s)
    --- PASS: TestStackCreateAndList/List_without_options (2.41s)
    --- PASS: TestStackCreateAndList/List_with_pagination (2.57s)
    --- PASS: TestStackCreateAndList/List_with_sort_options (3.39s)
=== RUN   TestStackReadUpdateDelete
--- PASS: TestStackReadUpdateDelete (10.94s)
PASS
ok  	github.com/hashicorp/go-tfe	24.318s

[sebasslash]

Minor things. Should we also add a changelog entry?

[github-actions]

Reminder to the contributor that merged this PR: if your changes have added important functionality or fixed a relevant bug, open a follow-up PR to update CHANGELOG.md with a note on your changes.