Plugins: Refactor cleaning of call resource response headers (#67145)

First part of #66889 moving cleaning of call resource response headers 
within plugin management client.

pkg/services/pluginsintegration/clientmiddleware/resource_response_middleware_test.go

@@ -0,0 +1,41 @@
+package clientmiddleware
+
+import (
+	"context"
+	"net/http"
+	"testing"
+
+	"github.com/grafana/grafana-plugin-sdk-go/backend"
+	"github.com/grafana/grafana/pkg/plugins/manager/client/clienttest"
+	"github.com/stretchr/testify/require"
+)
+
+func TestResourceResponseMiddleware(t *testing.T) {
+	t.Run("Should set proxy response headers when calling CallResource", func(t *testing.T) {
+		crResp := &backend.CallResourceResponse{
+			Status: http.StatusOK,
+			Headers: map[string][]string{
+				"X-Custom": {"Should not be deleted"},
+			},
+		}
+		cdt := clienttest.NewClientDecoratorTest(t,
+			clienttest.WithMiddlewares(NewResourceResponseMiddleware()),
+			clienttest.WithResourceResponses([]*backend.CallResourceResponse{crResp}),
+		)
+
+		var sentResponse *backend.CallResourceResponse
+		sender := callResourceResponseSenderFunc(func(res *backend.CallResourceResponse) error {
+			sentResponse = res
+			return nil
+		})
+
+		err := cdt.Decorator.CallResource(context.Background(), &backend.CallResourceRequest{
+			PluginContext: backend.PluginContext{},
+		}, sender)
+		require.NoError(t, err)
+
+		require.NotNil(t, sentResponse)
+		require.Equal(t, "sandbox", sentResponse.Headers["Content-Security-Policy"][0])
+		require.Equal(t, "Should not be deleted", sentResponse.Headers["X-Custom"][0])
+	})
+}