1. Consolidate config options for handler detection in the base class
AbstractMethodMessageHandler with sub-classes like RSocketMessageHandler
now only setting the handler predicate by default (e.g. @Controller).
2. Remove autoDetection flag in favor of just having the mutually
exclusive handler Predicate<Object> vs manually registered List<Object>.
Or if both are desired for some reason, then manually register first,
and set the predicate second.
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