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

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

If we extend the [create changelog](https://github.com/FAForever/fa/blob/develop/.github/workflows/changelog.yaml) 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](https://github.com/FAForever/fa/tree/develop/changelog/snippets/sections), and specifically the [footer](https://github.com/FAForever/fa/blob/develop/changelog/snippets/sections/template-footer.md) and the [header](https://github.com/FAForever/fa/blob/develop/changelog/snippets/sections/template-header.md) to make them more representative somehow, but that can be done in a separate pull request.