[flagd-provider] fix non-conformant config options

[toddbaert]

Currently the flagd provider doesn't default to unlimited retries, and doesn't throw immediately if it can't start. It also doesn't configure the retry policy of other providers (as in Go and Java), as well as other features.

Definition of done:

• update the flagd provider to default to 0 (infinite) retries
• reject immediately if the provider cannot connect, but continue to try to connect
• add "grace-period"
• add gRPC retry policy
• add "retryable status codes"
• verify other defaults/params
• update gherkin suite to verify conformance

See:

• docs: fatal codes, re-init, and retry policy flagd#1818 and existing documentation
• https://github.com/open-feature/go-sdk-contrib/tree/main/providers/flagd
  • Infinite retry to establish connection to FlagSyncService in Flagd golang provider go-sdk-contrib#756
  • fix(flagd): configurable retry backoff after each sync cycle error (#756) go-sdk-contrib#806

[toddbaert]

Closed in favor of #1096