unique() filter doesn't work chained after a filter that return an async generator.

This introduces a async variant of the filter transform the async
generator into a list to be able to apply the unique filter.

Fixes #1781

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.