[CovManager] Implement MozSourceCodeProvider for mozilla-specific repository

Currently, we do not support displaying coverage for bindings since these files are generated at build time. This is particularly import for Firefox builds as the binding coverage demonstrates a direct correlation between the WebIDL and the interfaces they describe. As this is process is specific to mozilla, we'll subclass the HGSourceCodeProvider as MozSourceCodeProvider.

The current plan for implementing this is:

1. Add method to base SourceCodeProvider class for performing initial preprocessing steps
2. When parsing incoming coverage data, call preprocessing
   • https://github.com/MozillaSecurity/FuzzManager/blob/7003587c04245353068dd0197ec0b4f2bfef5689/server/covmanager/tasks.py#L13-L23
3. Extract all generated bindings and store in location associated with rev
4. When fetching source in MozSourceCodeProvide.getSource, if hg cat fails check bindings
   • https://github.com/MozillaSecurity/FuzzManager/blob/7003587c04245353068dd0197ec0b4f2bfef5689/server/covmanager/SourceCodeProvider/HGSourceCodeProvider.py#L35-L43