Add support for Content Security Policy in Jenkins 2.539+

### What feature do you want to see added?

https://www.jenkins.io/changelog/2.539/ added Content Security Policy support.

This plugin should automatically allow the content it contributes.

### Upstream changes

_No response_

### Are you interested in contributing this feature?

Yes; although I'm still thinking about how to make that work well without breaking random other stuff. Suggestions/ideas welcome.

`simple-theme-plugin` has it easier, as that's more restrictive in what content can be provided. This plugin OTOH allows anything, which makes this more complicated.