Plugins: Introduced pluggable filter chain to be able to filter transport actions execution

It's now possible to inject action filters from plugins via `ActionModule#registerFilter` through the following code:

```
    public void onModule(ActionModule actionModule) {
          actionModule.registerFilter(MyFilter.class);
    }
```

Also made `TransportAction#execute` methods final to enforce the execution of the filter chain. By default the chain is empty though.

Note that the action filter chain is executed right after the request validation, as the filters might rely on a valid request to do their work.
Actually... I needed a special callback for the other method (`testTooManyContinueProcessing`) that doesn't require an operation. I don't want the many calls to be part of the Operation enum though cause it's quite a special case.