[Exporter] Add `List` operation for `users` service

[alexott]

Changes

Now we can list users and service principals directly, not only as groups dependency.

Tests

• [x] make test run locally
• [x] relevant change in docs/ folder
• [ ] covered with integration tests in internal/acceptance
• [ ] relevant acceptance tests are passing
• [ ] using Go SDK

[github-actions[bot]]

If integration tests don't run automatically, an authorized user can run them manually by following the instructions below:

Trigger: go/deco-tests-run/terraform

Inputs:

• PR number: 4204
• Commit SHA: f4eadcb5e1fc0359bc65f8d3df2c01d06acd5e49

Checks will be approved automatically on success.

[eng-dev-ecosystem-bot]

Test Details: go/deco-tests/11754609011