Expose Undici's ProxyAgent and setGlobalDispatcher within Node

Originally posted by @pimterry in https://github.com/nodejs/node/issues/42814#issuecomment-1134535017

As a motivating example: I'd love to automatically support global Fetch in https://www.npmjs.com/package/global-agent (drop-in lib to make all Node HTTP use the system proxy) but even though Node bundles Undici AFAICT this isn't possible without adding a full separate Undici dep to that lib, which would more than double its size, just [EDIT: largely] to ship code that's already present in Node itself.

This would be immediately solved if ProxyAgent and setGlobalDispatcher were exposed explicitly somewhere in future.

I've just done a quick test on main in Undici: exposing ProxyAgent & setGlobalDispatcher explicitly increases the Undici bundle size from 334.2kb to 336.0kb (+1.8kb / +0.5%).

Could these APIs be included and exposed in future? Agents for HTTP are a very core API that it would be useful to have usable out of the box, the equivalent functionality is usable OOTB for the legacy http module APIs, and 2kb is not a significant jump in bundle size for this functionality.

Start by tracing how Node exposes its bundled Undici functionality and where public HTTP APIs are defined. Verify the intended exposure points for ProxyAgent and setGlobalDispatcher, then add coverage showing they are usable through Node without a separate Undici dependency.