Add cross-signing feature flag

Fixes https://github.com/vector-im/riot-web/issues/11407

src/MatrixClientPeg.js

@@ -220,6 +220,16 @@ class MatrixClientPeg {
             identityServer: new IdentityAuthClient(),
         };
 
+        if (SettingsStore.isFeatureEnabled("feature_cross_signing")) {
+            // TODO: Cross-signing keys are temporarily in memory only. A
+            // separate task in the cross-signing project will build from here.
+            const keys = [];
+            opts.cryptoCallbacks = {
+                getCrossSigningKey: k => keys[k],
+                saveCrossSigningKeys: newKeys => Object.assign(keys, newKeys),
+            };
+        }
+
         this.matrixClient = createMatrixClient(opts);
 
         // we're going to add eventlisteners for each matrix event tile, so the