[BETA] Create a project-owned varset

[jbonhag]

Description

This adds an optional Parent relationship to a variable set, which allows setting either an organization or a project as the varset's parent. This is a feature that allows a variable set to be owned directly by a project.

Testing plan

1. An example of creating a project-owned varset is included in TestVariableSetsCreate
2. Without the feature enabled, the Parent relationship will be ignored by the backend, and will not be returned from the API.

Output from tests

❯ TFE_ADDRESS="<address>" TFE_TOKEN="<token>" ENABLE_BETA=1 go test ./... -v -run "TestVariableSets(Create|Read)"

?   	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   TestVariableSetsCreate
=== RUN   TestVariableSetsCreate/with_valid_options
=== RUN   TestVariableSetsCreate/when_options_is_missing_name
=== RUN   TestVariableSetsCreate/when_options_is_missing_global_flag
=== RUN   TestVariableSetsCreate/when_creating_project-owned_variable_set
--- PASS: TestVariableSetsCreate (2.04s)
    --- PASS: TestVariableSetsCreate/with_valid_options (0.39s)
    --- PASS: TestVariableSetsCreate/when_options_is_missing_name (0.00s)
    --- PASS: TestVariableSetsCreate/when_options_is_missing_global_flag (0.00s)
    --- PASS: TestVariableSetsCreate/when_creating_project-owned_variable_set (0.83s)
=== RUN   TestVariableSetsRead
=== RUN   TestVariableSetsRead/when_the_variable_set_exists
=== RUN   TestVariableSetsRead/when_variable_set_does_not_exist
=== RUN   TestVariableSetsRead/with_parent_relationship
--- PASS: TestVariableSetsRead (1.70s)
    --- PASS: TestVariableSetsRead/when_the_variable_set_exists (0.20s)
    --- PASS: TestVariableSetsRead/when_variable_set_does_not_exist (0.19s)
    --- PASS: TestVariableSetsRead/with_parent_relationship (0.18s)
PASS
ok  	github.com/hashicorp/go-tfe	4.039s

[netramali]

Should we add a comment to the CHANGELOG?

[ctrombley]

Should we add a comment to the CHANGELOG?

That would be most excellent! I'm planning on a release today so I'll try and get this change in if the timing works.

[netramali]

Other than the changelog looks good! Let me know if you need another approval :)

[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.