flatpak: add deploy step for Flathub

[stefanb2]

When commits are pushed to branch "flathub-release" an additional step will be executed to deploy the Flatpak build to Flathub.

• requires secret FLATHUB_TOKEN to be added to GitHub configuration
• add act-cli files needed for "push" event testing to .gitignore

[stefanb2]

@bkueng the value for flat-manager-url is a guess. I have been unable to find any documentation for the API URL of the Flathub repository.

[bkueng]

Hmm quickly looking at these:

• https://github.com/flathub/flathub/issues/3281
• https://github.com/flatpak/flatpak-github-actions/issues/69
• https://discourse.flathub.org/t/flathub-in-2023/3808#direct-uploads-12

It seems not to be generally available yet.