Add async log variants to structlog.stdlib.BoundLogger

[hynek]

Summary

This seems the better approach than AsyncBoundLogger.

Pull Request Check List

• Added tests for changed code.
  • The CI fails with less than 100% coverage.
• New APIs are added to typing_examples.py.
• Updated documentation for changed code.
  • New functions/classes have to be added to docs/api.rst by hand.
  • Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
    Find the appropriate next version in our __init__.py file.
• Documentation in .rst and .md files is written using semantic newlines.
• Changes (and possible deprecations) are documented in the changelog.
• Consider granting push permissions to the PR branch, so maintainers can fix minor issues themselves without pestering you.