Bluetooth: Mesh: Populate Bridge Config Server

Adds implementation for the Bridge Configuration Server model.

Updates `brg_cfg` module to add sanity check for
bt_mesh_brg_cfg_tbl_remove() API. Also, updates the unit test
accordingly.

Adds documentation for the Bridge Configuration Server model

Signed-off-by: Omkar Kulkarni <omkar.kulkarni@nordicsemi.no>