Bluetooth: Mesh: allow user to define BLOB ID for bt_mesh_dfu_cli_send

User can now use self-defined BLOB ID for transfer instead of using
random one each time. This allows to send multiple FW Update Start
messages with same BLOB ID, for example to resume previous one.

Signed-off-by: Krzysztof Kopyściński <krzysztof.kopyscinski@codecoup.pl>