Introduce submessage view accessors for arbitrary depths

Before this CL, we weren't generating view accessors for messages with depth > 1.
We weren't getting to message::GetterForViewOrMut because we were detecting FieldDescriptor::TYPE_MESSAGE and bailing out.

That check has now been expunged, and we now properly emit the right view, even for messages embedded within messages.

Added tests for:
- another level of submsg access depth
- accessing a message declared outside of the current message (that is, not a direct nested message within the same message)
- accessing the accessor of the accessor of the accessor of a recursively defined message

PiperOrigin-RevId: 588460599