Add gRPC support to mains plugin

[raphael]

This PR enhances the mains plugin to support gRPC servers in addition to HTTP servers, addressing #209.

Summary

The mains plugin now intelligently generates server code based on what transports are actually defined in the Goa design:

• HTTP-only services: Generates HTTP server initialization only
• gRPC-only services: Generates gRPC server initialization only
• Hybrid services: Generates both HTTP and gRPC servers
• Metrics server: Always generated on a separate port for health/metrics/debug endpoints

Key Changes

1. Conditional Transport Generation

• Scans the Goa DSL to detect which transports (HTTP/gRPC) are actually used
• Only generates server initialization code for transports that are defined
• Conditionally imports transport-specific packages (e.g., gorilla/websocket, google.golang.org/grpc)

2. Fixed Bugs

• Duplicate package/import: Removed duplicate package main and import blocks from template (now handled by codegen.Header())
• WebSocket detection: Fixed incorrect streaming detection logic (Stream value of 0 means no streaming, not NoStreamKind=1)

3. gRPC Server Support

• Proper gRPC server initialization with OpenTelemetry instrumentation
• gRPC interceptors for logging and debug (matching HTTP middleware pattern)
• Server reflection for easier testing with grpcurl
• Graceful shutdown support

4. Import Management

• Base imports always included (context, flag, fmt, http, os, signal, sync, syscall, time)
• OTel imports always included (for metrics server)
• net package only imported if gRPC is used
• google.golang.org/grpc packages only imported if gRPC is used
• gorilla/websocket only imported if streaming endpoints are defined

Example

For a service with both HTTP and gRPC endpoints:

var _ = Service("calc", func() {
    Method("add", func() {
        Payload(func() {
            Field(1, "a", Int)
            Field(2, "b", Int)
        })
        Result(Int)
        HTTP(func() { GET("/add/{a}/{b}") })
        GRPC(func() {})
    })
})

The generated main.go now includes:

• HTTP server on :8080
• gRPC server on :9090
• Metrics server on :8081
• Proper OTel instrumentation for both
• Graceful shutdown for both

Testing

Tested with:

• HTTP-only service ✅
• gRPC-only service ✅
• HTTP + gRPC hybrid service ✅
• Service with WebSocket streaming ✅
• Service with no streaming ✅

All generated code compiles and runs successfully.

Fixes #209