Glue: Return correlation config in GET /correlations (#55801)

* Fix getting and creating correlations config

* Fix tests

pkg/tests/api/correlations/correlations_create_test.go

@@ -272,6 +272,7 @@ func TestIntegrationCreateCorrelation(t *testing.T) {
 		require.Equal(t, description, response.Result.Description)
 		require.Equal(t, label, response.Result.Label)
 		require.Equal(t, "fieldName", response.Result.Config.Field)
+		require.Equal(t, map[string]interface{}{"expr": "foo"}, response.Result.Config.Target)
 
 		require.NoError(t, res.Body.Close())
 	})

pkg/tests/api/correlations/correlations_read_test.go

@@ -77,7 +77,7 @@ func TestIntegrationReadCorrelation(t *testing.T) {
 		OrgId:     dsWithCorrelations.OrgId,
 		Config: correlations.CorrelationConfig{
 			Field:  "foo",
-			Target: struct{}{},
+			Target: map[string]interface{}{},
 		},
 	})