Add env var & socket support for flagd provider

[toddbaert]

• Add env var reading for configuration (see: https://github.com/open-feature/golang-sdk-contrib/blob/eabb362ad36cf032ff277ecfd91e584d096b00dd/providers/flagd/pkg/provider.go#L92)
• Add unix socket support
• Do NOT add HTTP support (GRPC only)

[beeme1mr]

https://github.com/open-feature/java-sdk-contrib/tree/main/providers/flagd