[FEATURE] Make gRPC sync's gRPC connection customizable

Requirements

Today https://github.com/open-feature/flagd/blob/main/core/pkg/sync/grpc/grpc_sync.go is very rigid in terms of how it configures the gRPC connection and its credentials. Should a sync API implementation require clients to set up connections using particular credential types, or things like custom headers, it's impossible.

My proposal to solve that would be to be extend the Sync struct to have an optional field for fully custom []grpc.DialOption that would be used by grpc.NewClient() call. This is flexible and reasonably future proof:

type Sync struct {
	(...)
+	GrpcDialOptionsOverride []grpc.DialOption
	(...)
}

func (g *Sync) Init(_ context.Context) error {
	(...)
+	if len(g.GrpcDialOptionsOverride) > 0 {
+		g.Logger.Debug("GRPC DialOptions override provided")
+		rpcCon, err = grpc.NewClient(g.URI, g.GrpcDialOptionsOverride...)
+	} else {
		// Current logic
	(...)
}

Initially, I would like to contribute functionality to the flagd Go SDK that would utilize this new option (e.g. by extending the provider with WithGrpcDialOptionsOverride() option). I will file an issue in the Go SDK once we agree on the proposed solution here.

Start with core/pkg/sync/grpc/grpc_sync.go, especially the Sync struct and Init method where grpc.NewClient() is called. Trace the current connection and credential setup, then verify that an optional []grpc.DialOption override can be used while preserving the existing path when no override is provided.