[flagd] Update the flagd provider to conform to the latest spec

[toddbaert]

Update the flagd provider to conform to the latest spec: https://flagd.dev/reference/specifications/providers/

For the Java provider, that means:

• implement retryGraceAttempts param:
  • defaults to 5
  • when provider disconnects, if reconnect attempts <= retryGraceAttempts, provider emits STALE
  • when reconnect attempts > retryGraceAttempts provider is emits ERROR
• implement metadata-fetch with in-process init:
  • call https://buf.build/open-feature/flagd/docs/main:flagd.sync.v1#flagd.sync.v1.FlagSyncService.GetMetadata simultaneously with connecting the sync stream
    • fail init if metadata call fails
  • add contextEnricher function param:
    • converts payload from sync-metadata to Evaluation context on init
    • stores the result from above and adds it with every evaluation, probably using a hook (Java implementation: https://github.com/open-feature/java-sdk-contrib/blob/4b8aa8559fea5b995b10902eeabf1e9d4876b7bf/providers/flagd/src/main/java/dev/openfeature/contrib/providers/flagd/FlagdProvider.java#L72C23-L72C39)
• implement gRPC (HTTP2) keepalive param
  • add keepalive option to gRPC connections
• implement streamDeadlineMs (defaults to 600000 millis)
  • only applies to event stream (RPC) and sync stream (in-process)
• review and align all configuration params as specified in configuration: https://flagd.dev/reference/specifications/providers/#configuration