Allow overriding Node's internal fetch

[matthieusieben]

This would solve...

As Node's release cycles are significantly longer than those of undici, it would be nice to be able to benefit from new features & fixes faster.

The implementation should look like...

node -r undici/overrideGlobals index.js

and/or (as suggested by @mcollina)

undici.__overrideGlobals()

I have also considered...

Writing an additional/external npm package to do the same. This would require to maintain the version of that package in sync with undici, which migh be annoying.

Additional context

Mixing Requests from Node with Requests from undici can result in unexpected behaviors. This would mitigate that by allowing to mix those, at least for the specific version of undici that gets imported.

[mcollina]

I'm +1 to this approrach.