[YCQL] Prevent DROP TYPE for the used YCQL type

[Anagha-Damodaran]

Jira Link: DB-6394

Description

The current functionality allows for dropping a User-Defined Type (UDT) even if it is still being referenced. This can lead to unexpected behavior and errors in applications that rely on the UDT. To prevent this, it would be beneficial to add constraints to the DROP TYPE command to prevent dropping a UDT that is still being used.

Implementing such constraints can ensure that dropping a UDT only happens when it is safe to do so, and it can provide better control over the system's data types. This can help prevent errors and ensure that the system remains stable and reliable.

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[m-iancu]

Adding a repro here. It is working correctly for top-level UDTs, but it looks like nested/frozen UDTs are not properly checked.

Set up a UDT

ycqlsh> create keyspace sample;
ycqlsh> use sample;
ycqlsh:sample> create type tp(a int, b text);

Top-level UDT test

ycqlsh:sample> create table tbl(k int primary key, v1 tp, v2 int);
ycqlsh:sample> insert into tbl(k, v1, v2) values (1, { a : 2, b : 'abc'}, 3);
ycqlsh:sample> drop type tp;
InvalidRequest: Error from server: code=2200 [Invalid query] message="Invalid Request. Cannot delete type 'sample.tp'. It is 
used in column v1 of table tbl (master error 29)
drop type tp;
          ^^
 (ql error -316)"

^ correctly disallows dropping the UDT.

Frozen UDT test

ycqlsh:sample> drop table tbl;
ycqlsh:sample> create table tbl2(k int primary key, v1 frozen<tp>, v2 int);
ycqlsh:sample> insert into tbl2(k, v1, v2) values (1, { a : 2, b : 'abc'}, 3);
ycqlsh:sample> drop type tp;
ycqlsh:sample> select * from tbl2;

 k | v1                | v2
---+-------------------+----
 1 | tp(a=2, b=u'abc') |  3

^this should fail but succeeds.
Note that the table is still usable because the type information is stored expanded in the table schema (not just a reference to the type). So this is only an issue when we try to do backups.