* ci: adding 'e2e' GH workflows

The flow contains of multiple steps:

    * build 'query-service' and 'frontend' images and push them to the image registry
    * deploy a disposable k3s cluster
    * deploy the app on the cluster
    * set a tunnel to allow accessing the UI from the web browser
Signed-off-by: Yoni Bettan <ybettan@redhat.com>

* ci: requiring the 'ok-to-test' label for running some workflows

As of now, the 'e2e' workflow will require the 'ok-to-test' label in
order to get triggered.

In addition to that, on each change to the PR, Github will remove the
label from it and it will be required again.
Signed-off-by: Yoni Bettan <ybettan@redhat.com>