Bluetooth: shell: Add OBEX commands for GOEP

Add commands for GOEP to test OBEX features.

Add command `alloc-buf` and `release-buf` to allocate and release TX
buffer.

Add command set `add-header` to add the OBEX headers to allocated TX
buffer.

Add command set `client` to send OBEX client requests with allocated
TX buffer.

Add command set `server` to send OBEX responses with allocated TX
buffer.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>