Support custom badges

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What does this MR do and why?

This MR introduces custom badges. In current implementation you can change key_text but not value_text, color, etc.

https://gitlab.com/gitlab-org/gitlab/badges/main/custom.svg?key_text=Hello&value_text=World

this way you could create a latest tag badge, which is currently not possible

https://example.gitlab.com/%{project_path}/-/badges/custom.svg?key_text=Latest+Tag&value_text=%{latest_tag}

that might look like this:

References

• Support modification of badge text value #519816 (closed)
• Support creating static badge images #388108 (closed)
• Latest tag placeholder for badges #442124 (closed)