Spring Allow registration of RSocket metadata extractors

Prior to this commit, customizing the extraction of RSocket metadata from frames would require developers to override the default MetadataExtractor while configuring RSocketStrategies. This touches on many infrastructure parts, whereas the goal is just to configure an extra metadata entry extractor using already configured codecs.

This commit adds a way to register metadata entry extractors on the RSocketStrategies builder with a Consumer-based API.