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.
Closes gh-23645
848804a227