Makes ExtBot.callback_data_cache readonly and optional. In particular, the CDC is now only instantiated if requested. This is a preparation for making the corresponding 3rd party dependency optional.
Also removes ExtBot.arbitrary_callback_data.

Checklist for PRs

• [u] Added .. versionadded:: version, .. versionchanged:: version or .. deprecated:: version to the docstrings for user facing changes (for methods/class descriptions, arguments and attributes)
• Created new or adapted existing unit tests
• Documented code changes according to the CSI standard
• Added myself alphabetically to AUTHORS.rst (optional)
• Added new classes & modules to the docs and all suitable __all__ s