Refactor read-only iceberg table errors

[sfc-gh-abozkurt]

Remove unnecessary read only iceberg table errors. postgresIsForeignRelUpdatable should handle all dml errors.
We should throw error only for alter and truncate commands.

[sfc-gh-abozkurt]

we already do not push down such cases at IsCopyFromPushdownable.

[sfc-gh-okalaci]

why do we need this change?

[sfc-gh-abozkurt]

IsWritableIcebergTable is correct one after refactor. if block, which puts iceberg files to deletion queue, should only apply to writable iceberg tables.

[sfc-gh-okalaci]

I'm still not very much convinced that we get rid of ErrorIfReadOnlyIcebergTable, and introduce the same function in different places.

Can you remind me what is the reason not adjusting this one? I think having a single error message is even cleaner. For example, Postgres give a single error in read-only mode, which is a cleaner signal in the long term, compared to multiple different errors.

[sfc-gh-abozkurt]

We error only for read-only iceberg tables at alter hook. We error for both read-only iceberg and pg_lake tables at truncate.

Question: Shouldn't we also throw error for read-only pg_lake tables during alter commands? If so, we can create a more generic error function that can be reused for all modification cases.