Added gRPC authentication and authorization infrastructure

[Vikramarjuna]

Add gRPC Authentication and Authorization Infrastructure

Why

Enable gRPC support for Dominator services by sharing SRPC's existing authorization logic.

What

Adds core gRPC auth infrastructure:

• lib/grpc/api.go: Unary and streaming auth interceptors, Conn type for handlers to access auth info, ServiceOptions for registering public/unauthenticated methods
• lib/srpc/api.go: AuthConn interface (implemented by both SRPC and gRPC connections), GetDefaultGrantMethod(), GetPermittedMethods(), AllowMethodPowers()
• lib/srpc/server.go: Exported GetAuth() and CheckAuthorization() so gRPC can reuse the same 4-level authorization hierarchy

gRPC method names (/package.Service/Method) are translated to SRPC format (Service.Method) for RBAC compatibility.

Metrics stubs are included - actual metrics tracking comes in a later PR.

What Follows

• Streaming support (StreamingConn interface)
• Metrics + REST gateway + combined HTTP/gRPC server
• Adding support for VM creation, deletion and listing in gRPC and REST calls