Allow Paths in http proxy

[rohan-datar]

Currently, http proxies for only support a http://<host>:<port> scheme.
From the docs:

When specifying a proxy address, it must have scheme http, and may not contain anything other than a host and port.

Trying to proxy pkl packages through something like Artifactory Remote Repositories does not work as they add a path to the proxy url which is not supported and throws a Java exception when it attempts to resolve.

Allowing proxies to have a http://<host>:<port>/path/to/proxy would be very helpful

[HT154]

Pkl's HTTP proxy support is for CONNECT-based HTTP forward proxies. These work very differently from something like an Artifactory remote repo, which is a read-through caching reverse proxy.

Supporting mirroring via such a reverse proxy requires some additional handling and is being discussed in SPICE-0002.