Allow overriding channel name in endpoint

[taksan]

We do have a single channel to receive notifications from multiple repos. It would be nice if we could override de target channel passing a query parameter, for example

[ChristopheKar]

We would also love to have this option. A quick and slightly dirty solution to this could be to use the Gitlab secret token that you have the possibility to setup when creating the Webhook. Then, this line could be modified in GitLabEndpoint.ts to instead read headers['X-Gitlab-Token'] as the roomName, of course with some or condition to revert to the default roomName in case the header key is undefined. I would create this pull request but I'm having trouble testing it and deploying it locally through rc-apps deploy.

[metalcated]

Yeah I'd like this one too! :)