feat: add ability to DROP custom types

[agavra]

Description

Drops a previously registered type. This does not affect any previously issued commands or custom types (e.g. if I have CREATE TYPE B AS STRUCT<A> and A is STRING, dropping A will not affect B, which remains STRUCT<STRING>)

Testing done

• Unit testing
• E2E Manual Testing (after merging feat: add SHOW TYPES to list all custom types #3280)

ksql> CREATE TYPE ADDRESS AS STRUCT<number INTEGER, street VARCHAR, city VARCHAR>;

 Message
-------------------------------------------------------------------------------------------------
 Registered alias ADDRESS with SQL type STRUCT<`NUMBER` INTEGER, `STREET` STRING, `CITY` STRING>
-------------------------------------------------------------------------------------------------
ksql> SHOW TYPES;

 Type Name | Schema
----------------------------------------------------------------------------------
 ADDRESS   | STRUCT<NUMBER INTEGER, STREET VARCHAR(STRING), CITY VARCHAR(STRING)>
----------------------------------------------------------------------------------
ksql> DROP TYPE ADDRESS;

 Message
------------------------
 Dropped type 'ADDRESS'
------------------------
ksql> SHOW TYPES;

 Type Name | Schema
--------------------
--------------------

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[spena]

LGTM