Implements #13950

This PR unifies SHOW/DESCRIBE for tables and for queries. Instead of having a separate code path for when DESCRIBE [table] is called that directly accesses the underlying TableCatalogEntry - we now instead traverse the query plan in DESCRIBE [query] to retrieve constraint information. As a result, we now display constraint information also for queries like DESCRIBE SELECT * FROM tbl, or DESCRIBE SELECT col_name FROM tbl.

In addition, this PR also makes replacement scans work with describe - i.e. we can now run DESCRIBE "file.parquet" or DESCRIBE df directly.