Feature Request: Allow `getProxyForUrl` to be asynchronous

[mook-as]

(This request is for proxy-agent)

Currently, GetProxyForUrlCallback returns a string synchronously:

https://github.com/TooTallNate/proxy-agents/blob/dd3b98f0f0bf1da83a58c3330415805568b297f6/packages/proxy-agent/src/index.ts#L25

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!

[TooTallNate]

Sounds reasonable. I'd accept a PR.