knative - Content Extensions - Support invocation of non-standard extensions

With OpenWhisk, a web action can be invoked using a URL that is structured as follows:

```https://{APIHOST}/api/v1/web/{QUALIFIED ACTION NAME}.{EXT}```

For example,

``` https://${APIHOST}/api/v1/web/guest/demo/hello.http```

The fully qualified name of an action consists of three parts: the namespace, the package name, and the action name.

With Knative, a web action can be invoked using a Host and URL such as:

```curl -H "Host: nodejs-helloworld.default.example.com" -X POST http://<$IP_ADDRESS>/```

https://github.com/apache/incubator-openwhisk/blob/master/docs/webactions.md#additional-features

With OpenWhisk, the desired content type (one of .json, .html, .http, .svg or .text) can be specified as the extension to the action name. For example, an action `/guest/demo/hello` is referenced as `/guest/demo/hello.http` to receive an HTTP response back.

But with Knative, there is no means to specify such desired content type in terms of extension to the action name as the URL is common for all the actions without any action name ```http://<$IP_ADDRESS>/```.

We need to design this and find a better way to support content extensions with knative.