Add some webhook support github <-> testmachinery internal

[dguendisch]

What would you like to be added:
Some repos like https://github.com/gardener/gardener-extension-provider-openstack want to run integration tests via the tm-bot that target non-public facing networks.
For tests targeting non-public networks we have a testmachinery installation residing in said non-public networks (let's call it tm-internal).
However the tm-bot just reacts to github webhook events, hence public github cannot push events to the tm-bot from tm-internal.

We should add some mechanism that allows a tm-bot from tm-internal to react on public github events.
Possible options include:

• webhook queue (like gardener bot)
• tm-public to store received webhook events targetting internal networks; tm-internal to watch these received webhook events in tm-public and act accordingly

Why is this needed:
Allow public github repos to trigger integration tests targetting a non-public network.