Bluetooth: GATT: Add flag to indicate write command

Application may need to handle the write differently depending on the
write operation so this adds a flag called BT_GATT_WRITE_FLAG_CMD which
can then be checked by the callback, for instance one can respond with
BT_ATT_ERR_WRITE_REQ_REJECTED when that flag is not set which should
indicate to the client to use write command instead.

Fixes #11206

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>