- when instantiating a datasource, the datasource service checks if the
  plugin module exports Explore components, and if so, attaches them to
  the datasource
- Explore component makes all major internal pluggable from a datasource
  `exploreComponents` property
- Moved Prometheus query field to promehteus datasource and registered
  it as an exported Explore component
- Added new Start page for Explore, also exported from the datasource