net: conn_mgr: binding lock/unlock

Adds conn_mgr_binding_lock/conn_mgr_binding_unlock, which connectivity
implementations can call to lock or unlock the passed in connectivity
binding.

This allows connectivity implementations to safely make direct accesses
to binding data from outside conn_mgr_conn_api callbacks, and should
allow cleaner, more robust connectivity implementations to be written.

Signed-off-by: Georges Oates_Larsen <georges.larsen@nordicsemi.no>