Custom messages for specific actions

[heurtematte]

I would like otterdog to implement custom messages for specific actions depending on the organization configuration.

E.g: with this ticket https://gitlab.eclipse.org/eclipsefdn/helpdesk/-/issues/5267

When otterdog create a new repository, I would like otterdog to add a message to the PR after a repository newly created with something like:

The GitHub repo https://github.com/eclipse-<project_id>/some_repo has been created. Committers will gain access to it once the sync script runs (~2h).

Note: Implement a pre-action message or a post-action message, to order correctly messages and actions.

[netomi]

there is already a pre-add hook we could also add a post-add hook.

[matbmoser]

Thank you for the feature <3

[netomi]

should be live now, but it will only be seen once a repo has been created ofc.

[netomi]

You see how it looks like in this PR: eclipse-sumo/.eclipsefdn#4

I added an additional linebreak before the warning as otherwise it looks too packed.