did:web resolve bring-your-own-client

[KendallWeihe]

Our did:dht implementation enables consumers to bring-their-own-clients

But our did:web resolver just uses the native http.Get() call. Not sure exactly what this would look like, or if this even makes sense (also relevant to this ticket #94), but if we're enabling developer-defined clients in did:dht then we should do the same in did:web.

[KendallWeihe]

Relevant for configuring timeout settings, as none are set by default https://blog.cloudflare.com/the-complete-guide-to-golang-net-http-timeouts#client-timeouts