bluetooth: gatt: Add BT_GATT_CCC_WITH_WRITE_CB()

Add simple helper macro so users can define a CCCD with a write cb
without having to combine BT_GATT_CCC_MANAGED() and
BT_GATT_CCC_INITIALIZER() themselves.

Unlike the changed callback the write callback has a return value
that can be used to reject the write request.

Signed-off-by: Olivier Lesage <olivier.lesage@nordicsemi.no>