feat(ddl): allow drop generated column in table with schema registry

[yuhao-su]

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What's changed and what's your intention?

Before this pr, dropping any column from a table with schema registry is not allowed. We loose this constraint here.

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Allow drop generated columns from tables with schema registry.

ALTER TABLE t DROP COLUMN gen_col;

LIMITATION
dropping a generated column will also trigger refreshing schema.

Release note

If this PR includes changes that directly affect users or other significant modifications relevant to the community, kindly draft a release note to provide a concise summary of these changes. Please prioritize highlighting the impact these changes will have on users.

[yuhao-su]

Here is a problem: if we drop a generated column, a plan will be re-generated from the SQL definition, which means the schema will also be refreshed since we don't have purified select list. cc. @st1page

[st1page]

Here is a problem: if we drop a generated column, a plan will be re-generated from the SQL definition, which means the schema will also be refreshed since we don't have purified select list. cc. @st1page

Yes, that's a known issue, it will also happen when sink into table. Let's treat it as a separate issue and assume that the external schema (from the schema registry) will not change.