Feature Request: Allow `getProxyForUrl` to be asynchronous

(This request is for proxy-agent)

Currently, GetProxyForUrlCallback returns a string synchronously:

proxy-agents/packages/proxy-agent/src/index.ts

Line 25 in dd3b98f

type GetProxyForUrlCallback = (url: string) => string;

However, it may be necessary to resolve it asynchronously; for example Electron.Session.resolveProxy(url) returns a Promise<string>.

Since that is only used in async connect(), making that asynchronous shouldn't be too onerous; for backwards compatibility, it should be possible to:

Make it type GetProxyForUrlCallback = (url: string) => string | Promise<string>;
Change the single call site to const proxy = await Promise.resolve(this.getProxyForUrl(url));

Please let me know if that seems reasonable and I can attempt to whip up a PR. (If you think that it would be easier to implement the feature directly, that's excellent too.)

Thanks!

Sounds reasonable. I'd accept a PR.