Add gRPC API alongside JSON-RPC

[tac0turtle]

Summary

Introduce a gRPC server exposing the same blob and header APIs currently served over JSON-RPC. gRPC provides better performance characteristics (binary serialization, HTTP/2 multiplexing, streaming) which matters for high-throughput namespace indexing workloads.

Motivation

• Performance: Protobuf encoding is significantly faster and smaller than JSON-RPC payloads, especially for blob data which can be large
• Streaming: gRPC bidirectional streaming is a natural fit for Subscribe methods (headers, blobs)
• Type safety: Protobuf service definitions provide a strict contract and codegen for clients
• Ecosystem: Many Cosmos/Celestia ecosystem tools already use gRPC as the primary transport

Requirements

Proto definitions

• Define BlobService and HeaderService in .proto files
• Mirror the same methods as the JSON-RPC API:
  • Blob.Get, Blob.GetAll, Blob.Subscribe (server-stream), Blob.GetProof, Blob.Included
  • Header.GetByHeight, Header.Subscribe (server-stream), Header.LocalHead, Header.NetworkHead

Server

• gRPC server listening on a configurable port (separate from JSON-RPC)
• Add [grpc] address config field
• Reuse the same underlying store and sync engine — no logic duplication

Client (optional, for testing/tooling)

• Generated Go client from proto definitions
• CLI or test harness that can exercise the gRPC endpoints

Non-goals

• Replacing JSON-RPC — both transports coexist. JSON-RPC remains the default for ev-node compatibility.

References

• Design doc: test_plan.md