The commit introduces a new extension point, EnvironmentPostProcessor,
that can be implemented by classes that want to modify the
environment. Implementations of EnvironmentPostProcessor are loaded
via spring.factories and called in response to the
ApplicationEnvironmentPreparedEvent. Application listeners that wish
to work with the post-processed environment can continue to listen
to ApplicationEnvironmentPreparedEvent and order themselves to
run after EnvironmentPostProcessingApplicationListener.

Existing ApplicationListeners that modify the environment have,
where possible, been updated to implement EnvironmentPostProcessor
instead.

Closes gh-3737