[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
• review and align all configuration params as specified in configuration: https://flagd.dev/reference/specifications/providers/#configuration

[aepfli]

should we maybe split this into smaller tasks based on the todos - everything at once might be overwhelming, but all the todos as little chunks seems to me like good first issues ;)

[guidobrei]

We should also define the logging behavior during the grace period.
Info logs without exception details during that period should be sufficient to not be too noisy.

[aepfli]

Does this supersed #395?