Implement "SHOW CREATE TABLE" for external tables #2848

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Following on from #2529, it would be good to support external tables in "SHOW CREATE TABLE", as spark does.

Describe the solution you'd like
Implement TableProvider::get_table_definition for ListingTable, as in #2830. This will probably require implementing Display for DFStatement to convert it back into SQL, or generating SQL in register_listing_table.

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