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 <radu@cockroachlabs.com>