[feat](protocol, client, server): replace DTLS with gRPC for tunnel implementation

- Introduced gRPC-based tunnel design for bi-directional communication, replacing legacy DTLS transport.
- Added `HopGateTunnel` gRPC service with client and server logic for `OpenTunnel` stream handling.
- Updated client to use gRPC tunnel exclusively, including experimental entry point for stream-based HTTP proxying.
- Removed DTLS-specific client, server, and related dependencies (`pion/dtls`).
- Adjusted `cmd/server` to route gRPC and HTTP/HTTPS traffic dynamically on shared ports.

internal/protocol/pb/hopgate_stream_grpc.go

@@ -0,0 +1,119 @@
+package pb
+
+import (
+	"context"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+)
+
+// HopGateTunnelClient is the client API for the HopGateTunnel service.
+type HopGateTunnelClient interface {
+	// OpenTunnel establishes a long-lived bi-directional stream between
+	// a HopGate client and the server. Both HTTP requests and responses
+	// are multiplexed as Envelope messages on this stream.
+	OpenTunnel(ctx context.Context, opts ...grpc.CallOption) (HopGateTunnel_OpenTunnelClient, error)
+}
+
+type hopGateTunnelClient struct {
+	cc grpc.ClientConnInterface
+}
+
+// NewHopGateTunnelClient creates a new HopGateTunnelClient.
+func NewHopGateTunnelClient(cc grpc.ClientConnInterface) HopGateTunnelClient {
+	return &hopGateTunnelClient{cc: cc}
+}
+
+func (c *hopGateTunnelClient) OpenTunnel(ctx context.Context, opts ...grpc.CallOption) (HopGateTunnel_OpenTunnelClient, error) {
+	stream, err := c.cc.NewStream(ctx, &_HopGateTunnel_serviceDesc.Streams[0], "/hopgate.protocol.v1.HopGateTunnel/OpenTunnel", opts...)
+	if err != nil {
+		return nil, err
+	}
+	return &hopGateTunnelOpenTunnelClient{ClientStream: stream}, nil
+}
+
+// HopGateTunnel_OpenTunnelClient is the client-side stream for OpenTunnel.
+type HopGateTunnel_OpenTunnelClient interface {
+	Send(*Envelope) error
+	Recv() (*Envelope, error)
+	grpc.ClientStream
+}
+
+type hopGateTunnelOpenTunnelClient struct {
+	grpc.ClientStream
+}
+
+func (x *hopGateTunnelOpenTunnelClient) Send(m *Envelope) error {
+	return x.ClientStream.SendMsg(m)
+}
+
+func (x *hopGateTunnelOpenTunnelClient) Recv() (*Envelope, error) {
+	m := new(Envelope)
+	if err := x.ClientStream.RecvMsg(m); err != nil {
+		return nil, err
+	}
+	return m, nil
+}
+
+// HopGateTunnelServer is the server API for the HopGateTunnel service.
+type HopGateTunnelServer interface {
+	// OpenTunnel handles a long-lived bi-directional stream between the server
+	// and a HopGate client. Implementations are responsible for reading and
+	// writing Envelope messages on the stream.
+	OpenTunnel(HopGateTunnel_OpenTunnelServer) error
+}
+
+// UnimplementedHopGateTunnelServer can be embedded to have forward compatible implementations.
+type UnimplementedHopGateTunnelServer struct{}
+
+// OpenTunnel returns an Unimplemented error by default.
+func (UnimplementedHopGateTunnelServer) OpenTunnel(HopGateTunnel_OpenTunnelServer) error {
+	return status.Errorf(codes.Unimplemented, "method OpenTunnel not implemented")
+}
+
+// RegisterHopGateTunnelServer registers the HopGateTunnel service with the given gRPC server.
+func RegisterHopGateTunnelServer(s grpc.ServiceRegistrar, srv HopGateTunnelServer) {
+	s.RegisterService(&_HopGateTunnel_serviceDesc, srv)
+}
+
+// HopGateTunnel_OpenTunnelServer is the server-side stream for OpenTunnel.
+type HopGateTunnel_OpenTunnelServer interface {
+	Send(*Envelope) error
+	Recv() (*Envelope, error)
+	grpc.ServerStream
+}
+
+func _HopGateTunnel_OpenTunnel_Handler(srv interface{}, stream grpc.ServerStream) error {
+	return srv.(HopGateTunnelServer).OpenTunnel(&hopGateTunnelOpenTunnelServer{ServerStream: stream})
+}
+
+type hopGateTunnelOpenTunnelServer struct {
+	grpc.ServerStream
+}
+
+func (x *hopGateTunnelOpenTunnelServer) Send(m *Envelope) error {
+	return x.ServerStream.SendMsg(m)
+}
+
+func (x *hopGateTunnelOpenTunnelServer) Recv() (*Envelope, error) {
+	m := new(Envelope)
+	if err := x.ServerStream.RecvMsg(m); err != nil {
+		return nil, err
+	}
+	return m, nil
+}
+
+var _HopGateTunnel_serviceDesc = grpc.ServiceDesc{
+	ServiceName: "hopgate.protocol.v1.HopGateTunnel",
+	HandlerType: (*HopGateTunnelServer)(nil),
+	Streams: []grpc.StreamDesc{
+		{
+			StreamName:    "OpenTunnel",
+			Handler:       _HopGateTunnel_OpenTunnel_Handler,
+			ServerStreams: true,
+			ClientStreams: true,
+		},
+	},
+	Metadata: "internal/protocol/hopgate_stream.proto",
+}