jobs: use builtin to synthesize pending session job rows in vtable

[dt]

This adds a builtin to enumerate jobs pending creation in the current session. It then adds a UNION with a call to this builtin, rather than directly accessing the session in the Go code, to the body of the crdb_internal.jobs vtable, bringing the vtable closer to being just a pure query that can be replaced by a delegate or view.

Additionally, deprecated columns in the vtable, which were always null, are removed.

Release note: none. Epic: none.a