Resolve T365281 "/notify of sourceconfig reload"

Covers edge case when the library is missing source configs (currently 1 minute max) and the app generates enough events to overflow the queue (see comment in previous MR):

• Sets up consumers for StreamConfigFetcher which gets source configs.
• Provides way to pass configs into the Builder when initializing.
• Extracts configFetchRunnable into its own class.

Closes T365281