Add QuotaHandler to GCP proxy

The quota handler terminates connections when quota is exceeded.

The next CL will add instrumentation for quota related metrics.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=185042675

java/google/registry/proxy/quota/TokenStore.java

@@ -79,7 +79,7 @@ public class TokenStore {
   private final ScheduledExecutorService refreshExecutor;
   private final Clock clock;
 
-  TokenStore(QuotaConfig config, ScheduledExecutorService refreshExecutor, Clock clock) {
+  public TokenStore(QuotaConfig config, ScheduledExecutorService refreshExecutor, Clock clock) {
     this.config = config;
     this.refreshExecutor = refreshExecutor;
     this.clock = clock;