Is your feature request related to a problem? Please describe.

It's often useful to mix-and-match github requests with requests to other services, e.g. the Vercel API, various internal APIs, etc. etc.

Describe the solution you'd like

Provide a fetch argument, just like github, context, core, glob, io, exec etc.

Describe alternatives you've considered

This issue seems to be trying to achieve a similar thing: #272

I also tried using github.request(...) at first, but while you can hit non-GitHub APIs, it's impossible to override the authorization header, for example. If that's an issue, it's one for the octokit repo, though.

Additional context

@octokit/request already depends on node-fetch so by using the same version, it wouldn't even add to the bundled size of this action, I don't think.

I'd be happy to open a PR for this if it would be accepted.