Generator: API: Add the helper method for setting the X-Opaque-ID header

(cherry picked from commit 3f7b2e8)

Author: karmi

internal/cmd/generate/commands/gensource/generator.go

@@ -434,6 +434,20 @@ func (f ` + g.Endpoint.MethodWithNamespace() + `) WithHeader(h map[string]string
 		}
 	}
 }
+`)
+
+	// Generate methods for the X-Opaque-ID header
+	g.w(`
+// WithOpaqueID adds the X-Opaque-Id header to the HTTP request.
+//
+func (f ` + g.Endpoint.MethodWithNamespace() + `) WithOpaqueID(s string) func(*` + g.Endpoint.MethodWithNamespace() + `Request) {
+	return func(r *` + g.Endpoint.MethodWithNamespace() + `Request) {
+		if r.Header == nil {
+			r.Header = make(http.Header)
+		}
+		r.Header.Set("X-Opaque-Id", s)
+	}
+}
 `)
 }