unify behavior for ALTER SOURCE and ALTER TABLE for schema change

[BugenZhao]

Since #20203 (which resolves #12199), users are free to call [ADD | DROP] COLUMN for a table no matter whether its schema is derived from external system (i.e., schema registry).

However, the behavior is inconsistent when it comes to ALTER SOURCE, since they are on different code paths.

• ALTER SOURCE DROP COLUMN is generally unsupported.
• Only ALTER SOURCE REFRESH SCHEMA is supported for schema change on a source with schema registry.
• We may want to deprecate ALTER SOURCE FORMAT .. ENCODE .. (...) (introduced in feat: implement ALTER SOURCE xx FORMAT xx ENCODE xx (...) #14057)