Create a Github workflow to compile the snippets and convert the output into a lua-based changelog file

[Garanas]

Description

Needs: https://github.com/FAForever/fa/issues/6237

If we extend the create changelog workflow to accept a workflow_call event with an input to a specific branch then we can use it to generate a workflow based on the snippets of that branch.

Then we can create a new workflow that uses the adjusted changelog creation workflow, retrieves the artifact and converts it into a lua-based changelog file using https://github.com/FAForever/fa/issues/6237.

This changelog file would represent the open changes of the public testing branches.

We'll probably also need to update the default templates, and specifically the footer and the header to make them more representative somehow, but that can be done in a separate pull request.