feat: upstream/remote support custom http client

[hidu]

Now, upstream/remote uses *http.Client to send requests,
but it's difficult when discovery is not by domain

[kolesnikovae]

Thank you for the contribution @hidu! Could you please clarify the problem you've encountered? I'm asking because if this is something common that others may experience as well, we could probably extend the configuration options at the higher level.

[hidu]

I deploy Pyroscope on a cluster.
Client discovers the cluster through a naming service, which provides an IP list (and port) instead of domain names.
As instances of Pyroscope are subject to change, the IP list in the naming service will also change accordingly.

I also expect to use the custom client to print request/response logs, just like other clients in the Client APP.