Suggestion: Proxying to another JSR API

[jollytoad]

To allow the use of private or local registries the JSR API should be capable of proxying to another instance, eg the public one.

I wanted to run a local JSR so that I could preview my package and get it into shape before publishing to the public JSR, but having to also publish dependencies (such as @std packages) is a real pain. Ideally my local JSR could proxy to the public one to avoid this. This would also open up the opportunity to run a private JSR, or even a JSR for testing packages in other runtimes (#179).