Support for unregistering bt_conn callbacks

[mtpr-ot]

Currently it's not possible to unregister bt_conn callbacks registered with bt_conn_cb_register(cb). This has the disadvantage that each callback need to have individual run-time switching built in, in order to disable.

Registering the same callback twice also has the sideeffect of generating double callbacks - which in some cases will cause unexpected behavior.

A new function bt_conn_cb_unregister(cb) should be implemented to allow unregistering a callback.

In a discussion about this feature, in #18237, the following was also noted:

• Callback list should be re-implemented using sys_slist_t
• Semaphore protection of list manipulation should be implemented, as these may occur from different threads