feat: Support company SOCKS/http proxies

[eseidel]

Some times users report that shorebird doesn't work on their company networks.

I actually don't know if we support proxies today? We use package:http, which might have some built in support?

If we don't, we should add support.

[mrzbn]

is there a support for using shorebird with proxy. currently our users in Iran can't get their app updated because of restrictions

[eseidel]

We do not currently have any official proxy support, no.