Consider re-allowing to refer to UDT outside of their keyspace

In CASSANDRA-6643 we decided to make UDT inaccessible outside of their keyspace of definition. Doing so mainly has the advantage that when we drop a keyspace, we don't have to worry its UDT being used in another keyspace. However, this directly conflict with functions (UDF) being global: we can't have functions working on UDT if functions are global and we don't allow UDT access outside their keyspace. Which, I believe, leave us with the following possible options:

1. we make UDT accessible anywhere (though their fully qualified name).
2. we don't support functions on UDT at all.
3. we make functions keyspace-scoped, either always, or only if they apply to UDT.
4. we revert CASSANDRA-6438 and make UDT global.

In a perfect world I would lean towards 4: the arguments to make UDT keyspace-scoped where not wrong per-se but weak in hindsight given the other options here. It is however basically too late: changing it would be a breaking change so we can't reasonably change this post-2.1.0, and while it's not released yet, it's not a change we can make without substantially delaying the final.

Option 2 feels rather lame in my book.

Option 3 feels pretty messy. Having 2 types of UDF, some keyspace-scoped and some that are not would be super confusing. Saying that ll UDF are keyspace-scoped feels limiting, and we would still be somewhat inconsistent with our existing hard-coded functions that are global.

Which leaves option 1 which might be the most pragmatic. Having to check that UDTs are not used before allowing keyspace drop don't sound like a huge deal to me.