Allow registering serialized lambda metadata through RuntimeHints

[odrotbohm]

If you need to declare GraalVM metadata for serialized lambdas, you currently would need that through a custom reachability-metadata.json in your application (example here). It would be nice if RuntimeHints allowed that through the API.

[snicoll]

See https://www.graalvm.org/latest/reference-manual/native-image/metadata/#specifying-metadata-with-json