Add `get_table_comment` method to inspection stub

[hsheth2]

See https://docs.sqlalchemy.org/en/14/core/reflection.html#sqlalchemy.engine.reflection.Inspector.get_table_comment

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[tower111]

when i use clickhouse dataset it can get properties

But mysql cann‘t get !

[hsheth2]

This is clearly never going to be merged. Closing for now.