Add spacelift_contexts data source

Type of change

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

[x] This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
[x] Pull Request is no longer marked as "draft"
[x] Reviewers have been assigned
[x] Changes have been reviewed by at least one other engineer
[x] The target branch is future unless the change is going directly into production