25126: opt: support for EXPLAIN r=RaduBerinde a=RaduBerinde
#### opt: introduce and build ExplainOp
The ExplainOp must remember the physical props required by the
underlying query.
For now we only support `EXPLAIN (VERBOSE)`.
Release note: None
#### opt: execbuild support for EXPLAIN
Release note: None
#### opt: remove exec-explain
Remove the `exec-explain` directive and convert all the tests to exec
an EXPLAIN statement. Add `hide-columns` option to suppress the header
with the column names and types, for cleaner output (and to avoid a
ton of diffs in this change).
Release note: None
Co-authored-by: Radu Berinde <[email protected]>