Fix DataSourceMetadata location

This commit complements 3dc932db8 and fix the package location of
DataSourceMetadata.

Fixes gh-1013
Reuse data source validation query in health endpoint

This commit improves DataSourceMetadata to expose the validation
query. This can be used by DataSourceHealthIndicator as the query
to use instead of "guessing" which query could be applied according
to the database type.

Fixes gh-1282