Provide Spring gRPC DiscoveryClient integration

Description

spring-projects/spring-grpc issue #341 requested support for resolving gRPC client targets such as discovery:///service-name through Spring Cloud DiscoveryClient.

A working prototype was implemented in spring-grpc, but Spring gRPC maintainers noted that this integration should live on the Spring Cloud side because of the dependency direction between Spring Boot, Spring gRPC and Spring Cloud:

• https://github.com/spring-projects/spring-grpc/issues/341#issuecomment-3705961699

Proposed Scope

Add a Spring Cloud module that bridges Spring Cloud service discovery with Spring gRPC by:

• resolving discovery:///service-name targets through DiscoveryClient
• refreshing resolved addresses when Spring Cloud publishes a HeartbeatEvent
• preferring the gRPC_port metadata entry and falling back to the regular service instance port
• optionally publishing spring.grpc.server.port as gRPC_port metadata through the Registration abstraction when metadata is mutable

Why This Belongs Here

The implementation depends on Spring Cloud abstractions such as:

• DiscoveryClient
• HeartbeatEvent
• Registration

while the gRPC side can stay limited to Spring gRPC extension points.

Prior Art / Prototype

The current prototype and discussion live here:

• Spring gRPC issue: https://github.com/spring-projects/spring-grpc/issues/341
• Spring gRPC PR: https://github.com/spring-projects/spring-grpc/pull/394

Question

Would Spring Cloud Commons accept this as:

1. a new module in this repository, or
2. a different Spring Cloud repository / subproject?

If this repository is acceptable, I can adapt the current prototype to match Spring Cloud Commons conventions and open a PR here.

Start with the prototype and discussion in spring-grpc issue #341 and PR #394, then review the DiscoveryClient, HeartbeatEvent, and Registration abstractions named here. Confirm the module location and Spring Cloud Commons conventions before adapting the integration; done means discovery:/// targets resolve, heartbeat changes refresh addresses, and gRPC_port metadata behavior is covered.