TF-9286 Add the enforced attribute to variable_set

[Netra2104]

Description

The variable sets has a new attribute called enforced to disable overwrites and hence go-tfe needs to be updated to accommodate the new attribute as well for POST and PATCH.

Testing plan

Integration tests.

External links

• RFC
• PRD
• Jira

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.

=== RUN   TestVariableSetsCreate
--- PASS: TestVariableSetsCreate (1.45s)
=== RUN   TestVariableSetsCreate/with_valid_options
    --- PASS: TestVariableSetsCreate/with_valid_options (0.36s)
PASS


=== RUN   TestVariableSetsUpdate
--- PASS: TestVariableSetsUpdate (1.61s)
=== RUN   TestVariableSetsUpdate/when_updating_a_subset_of_values
    --- PASS: TestVariableSetsUpdate/when_updating_a_subset_of_values (0.18s)
=== RUN   TestVariableSetsUpdate/when_options_has_an_invalid_variable_set_ID
    --- PASS: TestVariableSetsUpdate/when_options_has_an_invalid_variable_set_ID (0.00s)
PASS

[brandonc]

Perfecto

[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.