feat: use go:embed to embed templates (#1756)

tools/goctl/api/gogen/svc.tpl

@@ -0,0 +1,17 @@
+package svc
+
+import (
+	{{.configImport}}
+)
+
+type ServiceContext struct {
+	Config {{.config}}
+	{{.middleware}}
+}
+
+func NewServiceContext(c {{.config}}) *ServiceContext {
+	return &ServiceContext{
+		Config: c,
+		{{.middlewareAssignment}}
+	}
+}