new forgejo scaler

[cobak78]

This PR adds a new forgejo autoscaler with the same characteristics of the github autoscaler.
Here is the issue for this feature #6488

Another important discussions on this topic:

• The issue on forgejo with the new endpoint needed to make this autoscaler work. https://codeberg.org/forgejo/discussions/issues/241
• The accepted PR https://codeberg.org/forgejo/forgejo/pulls/6300
• The modifications on the forgejo runner to work as a scaled job. https://code.forgejo.org/forgejo/runner/pulls/423

Checklist

• When introducing a new scaler, I agree with the scaling governance policy
• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

[JorTurFer]

Thanks a lot for your contribution! I've left some comment inline. Additionally to them:

• Please, add an e2e test covering the scaler. You get more info about how to do it here
• Could you open a PR to docs adding the scaler in next version?
• All commits must be signed and that's why DCO check is failing, please rebase your branch signing all the commits (how to do that it's explained in the check page)

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