This pull request migrates testdata to coreplugin streaming capabilities,
this is mostly a working concept of streaming plugins at the moment,
the work will continue in the following pull requests.
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>