Flipt API V2 Tracking Issue

[markphelps]

This is the placeholder to link issues / feature requests that we'd like to see in Flipt V2 API (REST and GRPC)

1. V2 would allow for breaking changes from the V1 API
2. It would be a new API version in the path (ie REST API /api/v2) and would likely require new GRPC endpoints and .proto definitions
3. V2 namespaced GRPC clients would also need to be generated

Proposed Breaking Changes

• Don't allow empty rules to be created via API #367
• Convert to using GRPC Gateway v2 Marshaller #682