Add SetNumInstancesCommand in Nomulus tool to adjust the number of instances

for a given service and version at runtime.

Note that this CL only supports the adjustment for a given service and version. I will add another functionality to let this command be able to detect all non-live versions automatically and apply the adjustment.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=221092001

java/google/registry/util/AppEngineServiceUtils.java

@@ -37,4 +37,7 @@ public interface AppEngineServiceUtils {
 
   /** Returns a host name to use for the given service and version. */
   String getVersionHostname(String service, String version);
+
+  /** Set the number of instances at runtime for a given service and version. */
+  void setNumInstances(String service, String version, long numInstances);
 }