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)