Support custom name resolution in flagd providers

Summary

Introduce support for custom name resolution in flagd providers to enable better integration with service proxies like Envoy, facilitating more flexible and robust gRPC client configurations.

Background

In distributed systems, service proxies like Envoy are often used to manage traffic routing, load balancing, and observability. These proxies rely on the :authority header to route requests to the appropriate backend services. Some languages expose a mechanize for easily adding Custom Name Resolver (see gRPC docs), while others must be manually configured.

Requirements

### Tasks
- [x] https://github.com/open-feature/flagd/pull/1414
- [ ] Include custom name resolution RFC in docs
- [x] https://github.com/open-feature/flagd/pull/1424
- [x] https://github.com/open-feature/go-sdk-contrib/pull/587
- [x] https://github.com/open-feature/java-sdk-contrib/issues/995
- [ ] https://github.com/open-feature/js-sdk-contrib/issues/1187
- [ ] Custom gRPC resolver in Python
- [ ] Custom gRPC resolver in .NET

References

• gRPC Custom Name Resolve Spec
• Custom gRPC resolver E2E implementation

Start with docs/reference/specifications/proposal/rfc-grpc-custom-name-resolver.md and the referenced custom gRPC resolver E2E implementation. Review the remaining unchecked tasks for the documentation, Python, .NET, and JavaScript providers. Done means the RFC is documented and the listed provider resolver work is completed.