Add support for shim plugins

Refactor shim v2 to load and register plugins.
Update init shim interface to not require task service implementation on
returned service, but register as plugin if it is.

Signed-off-by: Derek McGowan <derek@mcg.dev>

services/server/server.go

@@ -142,13 +142,29 @@ func New(ctx context.Context, config *srvconfig.Config) (*Server, error) {
 
 		tcpServerOpts = append(tcpServerOpts, grpc.Creds(credentials.NewTLS(tlsConfig)))
 	}
+
+	// grpcService allows GRPC services to be registered with the underlying server
+	type grpcService interface {
+		Register(*grpc.Server) error
+	}
+
+	// tcpService allows GRPC services to be registered with the underlying tcp server
+	type tcpService interface {
+		RegisterTCP(*grpc.Server) error
+	}
+
+	// ttrpcService allows TTRPC services to be registered with the underlying server
+	type ttrpcService interface {
+		RegisterTTRPC(*ttrpc.Server) error
+	}
+
 	var (
 		grpcServer = grpc.NewServer(serverOpts...)
 		tcpServer  = grpc.NewServer(tcpServerOpts...)
 
-		grpcServices  []plugin.Service
-		tcpServices   []plugin.TCPService
-		ttrpcServices []plugin.TTRPCService
+		grpcServices  []grpcService
+		tcpServices   []tcpService
+		ttrpcServices []ttrpcService
 
 		s = &Server{
 			grpcServer:  grpcServer,
@@ -211,13 +227,13 @@ func New(ctx context.Context, config *srvconfig.Config) (*Server, error) {
 
 		delete(required, reqID)
 		// check for grpc services that should be registered with the server
-		if src, ok := instance.(plugin.Service); ok {
+		if src, ok := instance.(grpcService); ok {
 			grpcServices = append(grpcServices, src)
 		}
-		if src, ok := instance.(plugin.TTRPCService); ok {
+		if src, ok := instance.(ttrpcService); ok {
 			ttrpcServices = append(ttrpcServices, src)
 		}
-		if service, ok := instance.(plugin.TCPService); ok {
+		if service, ok := instance.(tcpService); ok {
 			tcpServices = append(tcpServices, service)
 		}