refactor(datasource): migrate to class based datasource (#6747)

* refactor(datasource): migrate to class based datasource

    A small experiment into what OOP/class based datasources might look like. Picked Cdnjs as it's the smallest & simplest.

    With this approach we can share common logic, like error handling, rate limiting, etc. between different datasources, instead of having to reimplement it each time we write a new datasource. Currently there's nothing shared, as it's only 1 datasource, but the interesting stuff will come with the 2nd datasource

* Apply suggestions from code review

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>

* remove renaming ClojureDatasource to datasource in tests

Co-authored-by: Rhys Arkins <rhys@arkins.net>
Co-authored-by: Michael Kriese <michael.kriese@visualon.de>