Add a new extension point SshExecuteCommandInterceptor

It allows plugin to intercept ssh commands within the SshScope.

It is added to address some limitations of the current
SshCreateCommandInterceptor extension point by allowing:
- to inject the SshSession within the interceptor (impossible with
  SshCreateCommandInterceptor being injected just before the
  SshContext is created)
- multiple plugins to bind it using DynamicSet bindings

Change-Id: If57cae8f82b48f6c2ae8f71fc6ff2027b51b9e98

java/com/google/gerrit/sshd/SshModule.java

@@ -101,6 +101,7 @@ public class SshModule extends LifecycleModule {
         .to(SshPluginStarterCallback.class);
 
     DynamicItem.itemOf(binder(), SshCreateCommandInterceptor.class);
+    DynamicSet.setOf(binder(), SshExecuteCommandInterceptor.class);
 
     listener().toInstance(registerInParentInjectors());
     listener().to(SshLog.class);