Fix shutdown on Ctrl+C for Python source stages

[dagardner-nv]

Description

• Ensure that Python sources which run indefinitely actually shutdown when requested by the pipline (ex: when the user hits Ctrl+C).
• The KafkaSourceStage already handled this properly, move the stop method from this class into the SingleOutputSource.
• Add request_stop and is_stop_requested methods to SingleOutputSource.
• Update all existing source stages which run indefinitely.
• Add new should_stop_fn constructor argument to DirectoryWatcher, sources which use the watcher (AutoencoderSourceStage & AppShieldSourceStage) pass in their is_stop_requested method allowing the watcher to know when to shut down.
• Move the RSS source generator method from the RSS source module into the RSSController, and update the RSSSourceStage to use the RSSController directly rather than the RSS module. This avoids the problem where modules require all config values to be JSON serializable, preventing a callback function from being passed to a module.

Closes #1837

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• I am familiar with the Contributing Guidelines.
• When the PR is ready for review, new or existing tests cover these changes.
• When the PR is ready for review, the documentation is up to date with these changes.

[dagardner-nv]

/merge