Add ability for a postprocessor to be applied to all resources

It's really convenient to do label/ annotation injection to all objects produced. Allow for a single post-processor block that returns a modified object for every source object that is given.

Exact interface TBD

Start by reviewing the repository's resource-processing entry points to identify where source objects are transformed. Define the post-processor interface and its scope with maintainers before implementation, since the issue leaves the exact interface unspecified. Done should include applying one post-processor to every produced resource and covering the behavior with tests.

Written by the indexing model from the issue text.