cmd/smithtest: add sqlsmith auto tester and github filer

smithtest is a CLI program for automating sqlsmith testing. It does normal
sqlsmith but via cockroach start-single-node, so found panics don't also
take down the test itself. Found errors are de-duplicated using existing
github issues. The reducer is used to minimize test repros. A new tab
is opened to github with most of the issue data filled in.

Release note: None