Converts the core testdata data source to use the SDK contracts and by that 
implementing a backend plugin in core Grafana in similar manner as an external one.

Co-authored-by: Will Browne <will.browne@grafana.com>
Co-authored-by: Marcus Efraimsson <marefr@users.noreply.github.com>
Co-authored-by: Ryan McKinley <ryantxu@gmail.com>