xds: Pre-add fallback to xds client pool accessor

When we implement A71, we're no longer going to have a single xds
client, but instead one per channel target. Add that parameter now, even
though it is unused, to avoid managing the (internal) API breakage when
we implement fallback.

xds/src/main/java/io/grpc/xds/InternalSharedXdsClientPoolProvider.java

@@ -32,7 +32,8 @@ public static void setDefaultProviderBootstrapOverride(Map<String, ?> bootstrap)
     SharedXdsClientPoolProvider.getDefaultProvider().setBootstrapOverride(bootstrap);
   }
 
-  public static ObjectPool<XdsClient> getOrCreate() throws XdsInitializationException {
+  public static ObjectPool<XdsClient> getOrCreate(String target)
+      throws XdsInitializationException {
     return SharedXdsClientPoolProvider.getDefaultProvider().getOrCreate();
   }
 }