Add support for users

[mwasilew2]

Description of the change

This is part of "User Management" set of features. It adds to the terraform provider support for managing user mappings. See the relevant clickup issue for more details.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• 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)

Related issues

Fix #1

Checklists

Development

• Lint rules pass locally
• The code changed/added as part of this pull request has been covered with tests
• All tests related to the changed code pass in development
• Examples for new resources and data sources have been added
• Default values have been documented in the description (e.g., "Dummy: (Boolean) Blah blah. Defaults to false.)
• If the action fails that checks the documentation: Run go generate to make sure the docs are up to date

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• Pull Request is no longer marked as "draft"
• Reviewers have been assigned
• Changes have been reviewed by at least one other engineer

[mwasilew2]

This is blocked on API changes necessary on the backend side

[mwasilew2]

blocked by: https://github.com/spacelift-io/backend/pull/5658

Note: the tests will be failing until the backend PR is deployed

[mwasilew2]

The backend PR was merged, the tests are passsing, this is ready for review

[michieldewilde]

Only the naming is an issue.

[mwasilew2]

ping @michieldewilde