Support fulldoc insert with subdoc upsert with LCB V4 API

SDK2 had a couple of tests that attempt fulldoc insert with subdoc upsert.

I can't see a way of doing this with the V4 API.

There is DICT_UPSERT, but you can't set the FULLDOC_INSERT flag on that.

There is FULLDOC_ADD, but that's not a DICT_UPSERT.

I think the use case for this is for XAttrs, but the test doesn't actually use it for that.

If it's still considered useful, we need to find a way to support it.