fix(github-runner-scaler) Add support for key authentication against GHES

[Eddman]

Current implementation is unable to use GitHub App private key to authenticate against custom API endpoint, eg. GitHub Enterprise Server (GHES).

Based on the https://github.com/bradleyfalzon/ghinstallation?tab=readme-ov-file#github-enterprise-example docs I'm adding support for GHES for authentication using GitHub App private key.

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)

Fixes #5387.

[JorTurFer]

/run-e2e github
Update: You can check the progress here

[JorTurFer]

Thanks for the fix! ❤️

[zroubalik]

Thanks for the fix!

Could you please open a short issue to describe the problem and update Changelog (Fixes in 2.13)?

Thanks!

[Eddman]

@zroubalik Hi, issue created, changelog modified.

When can we expect 2.13 to be released?

[zroubalik]

LGTM

[zroubalik]

@zroubalik Hi, issue created, changelog modified.

When can we expect 2.13 to be released?

Thanks!

Our plan is this Thursday.

[zroubalik]

/run-e2e github
Update: You can check the progress here