LE Audio: CSIP optional notification for Set Size

[Thalley]

Is your enhancement proposal related to a problem? Please describe.
The CSIS spec defines the Set Size as optionally notifiable. The CSIP Set Member implementation should support this optionally (using a Kconfig?) and the CSIP Set Coordinator should (optionally) subscribe to this if it is available.

Describe the solution you'd like
Add optional support for sending and receiving notifications for the Set Size. This also implies that the Set Member should have APIs to modify the Set Size at runtime.

Describe alternatives you've considered
N/A

Additional context
N/A