[flagd] add retry policy, and make configurable

[toddbaert]

Add a configurable retry backoff options to the flagd in-process provider, which allows to configure the retry policy of gRPC connections.

See equivalent implementation here (use the same variables and env-var names)

• grpc-js retry policy example: https://github.com/grpc/grpc-node/blob/164d14f28261ad1fbbc99d076fe850dad018b2bb/examples/retry/README.md?plain=1#L29-L56
• use the same policy as seen here: https://github.com/open-feature/go-sdk-contrib/pull/806/files#diff-431e1694cab09598ed0a65d71eb6f14d0352c3a208ed215ce03f01711bdde2b8R24-R45