Add support for disambiguating multiple responses for cache exporters

by adding IDs similar to output exporters.

Signed-off-by: a-palchikov <deemok@gmail.com>

Author: a-palchikov

api/services/control/control.pb.go

@@ -104,6 +104,9 @@ message CacheOptionsEntry {
 	// Attrs are like mode=(min,max), ref=example.com:5000/foo/bar .
 	// See cache importer/exporter implementations' documentation.
 	map<string, string> Attrs = 2;
+	// ID identifies this exporter.
+	// ID should be treated by the exporter as opaque.
+	string ID = 3;
 }
 
 message SolveResponse {

api/services/control/control.proto

@@ -2702,7 +2702,7 @@ func testSessionExporter(t *testing.T, sb integration.Sandbox) {
 		require.Equal(t, "foo", resp.Entries[1].Path)
 
 		exporterCalled = true
-		target.Add(filesync.WithFSSync(0, fixedWriteCloser(nopWriteCloser{outW})))
+		target.Add(filesync.WithFSSync("0", fixedWriteCloser(nopWriteCloser{outW})))
 		return []*exporter.ExporterRequest{
 			{
 				Type: ExporterOCI,