CSIS: Add support for parallel operations and make implementation thread-safe

[MariuszSkamra]

Is your enhancement proposal related to a problem? Please describe.
The current implementation disallows parallel operations on different set members. This might be problematic, as CSIS will be most likely used from different contexts.

Looking at the code, there is a static structure active_members that holds context. I guess the user might get into trouble when tries to use this API with multithreads and forbids concurrent operations on 2 sets.

[asbjornsabo]

Zephyr-bt F2F: Applies for all profiles in LA Audio.

[Thalley]

This is still valid, but needs to be split up into multiple issues, as fixing the entire LE Audio stack in a single go would be impossible

[Thalley]

The parallel operation support was added by #44867. The thread-safe part require a new issue.