open-feature/java-sdk

AI Summary: The task is to modify the Java OpenFeature SDK to expose the keys of the `ImmutableMetadata` object within `FlagEvaluationDetails`. This will allow clients to access and utilize the metadata associated with flag evaluations without resorting to reflection. The change should maintain the immutability of the metadata.

AI Summary: This task involves migrating the multi-provider functionality from the OpenFeature Java SDK's contrib repository into the main SDK repository. This includes deprecating or removing the multi-provider from the contrib repository and potentially simplifying the code now that it has access to SDK internals. The migrated multi-provider should be marked as experimental.