[Feat]: gRPC support — client and server

### Summary
Add built-in A2A support for the GRPC binding: a gRPC client implementing IA2AClient and an ASP.NET Core gRPC host that serves the same operations as today’s JSON-RPC / HTTP+JSON endpoints, backed by the existing IA2ARequestHandler pipeline.

### Background
The spec allows GRPC alongside JSON-RPC and HTTP+JSON. This repo already defines ProtocolBindingNames.Grpc and allows A2AClientFactory.Register(...), but there is no first-party gRPC implementation; README documents JSON-RPC + HTTP+JSON only. v1 JSON follows ProtoJSON conventions, which should align with a protobuf/gRPC contract if the protocol provides (or will provide) canonical .proto files.

### Goals
Client: IA2AClient over gRPC with parity for unary and streaming operations (e.g. SendStreamingMessage, SubscribeToTask), including sensible error mapping to A2AException / A2AErrorCode.
Server: Extension similar in spirit to MapA2A / MapHttpA2A that maps gRPC services to IA2ARequestHandler (reuse task/message/push-config behavior; do not fork business logic).
Packaging: Prefer a dedicated package (e.g. A2A.Grpc) so core users are not forced to reference Grpc.Net.* unless they need gRPC.
Quality: Integration tests; CI builds/tests on net8.0 / net10.0; validate Native AOT compatibility where required by this repo.
Open questions / dependencies
Source of truth for .proto (official A2A artifacts vs. maintained in-repo).
Exact streaming and error semantics per spec for gRPC vs. current SSE/JSON-RPC behavior.
### References
A2AClientFactory, ProtocolBindingNames.Grpc, MapA2A / MapHttpA2A, IA2ARequestHandler, README.md (protocol compatibility).

**Contributor note:** I am willing to implement and maintain this if the project wants to move forward with it.