Backend plugins: Refactor to allow shared contract between core and external backend plugins (#25472)

Refactor to allow shared contract between core and external backend plugins 
allowing core backend data sources in Grafana to be implemented in same 
way as an external backend plugin.
Use v0.67.0 of sdk.
Add tests for verifying plugin is restarted when process is killed.
Enable strict linting for backendplugin packages