[main] Plugin fixes (#57399)

* Plugins: Remove support for V1 manifests

* Plugins: Make proxy endpoints not leak sensitive HTTP headers

* Security: Fix do not forward login cookie in outgoing requests

(cherry picked from commit 4539c33fce5ef23badb08ebcbc09cb0cecb1f539)

Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>

pkg/util/proxyutil/reverse_proxy_test.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"github.com/grafana/grafana/pkg/infra/log"
+	"github.com/grafana/grafana/pkg/services/contexthandler"
 	"github.com/stretchr/testify/require"
 )
 
@@ -30,6 +31,11 @@ func TestReverseProxy(t *testing.T) {
 		req.Header.Set("Referer", "https://test.com/api")
 		req.RemoteAddr = "10.0.0.1"
 
+		const customHeader = "X-CUSTOM"
+		req.Header.Set(customHeader, "val")
+		ctx := contexthandler.WithAuthHTTPHeader(req.Context(), customHeader)
+		req = req.WithContext(ctx)
+
 		rp := NewReverseProxy(log.New("test"), func(req *http.Request) {
 			req.Header.Set("X-KEY", "value")
 		})
@@ -49,6 +55,7 @@ func TestReverseProxy(t *testing.T) {
 		require.Empty(t, resp.Cookies())
 		require.Equal(t, "sandbox", resp.Header.Get("Content-Security-Policy"))
 		require.NoError(t, resp.Body.Close())
+		require.Empty(t, actualReq.Header.Get(customHeader))
 	})
 
 	t.Run("When proxying a request using WithModifyResponse should call it before default ModifyResponse func", func(t *testing.T) {