Allow Basic Authentication in http_proxy

[avarei]

Currently only proxies without authentication are supported.

From the Docs:

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

So when trying to run pkl project resolve on a system with HTTP_PROXY env variable set I get an error:

Received unexpected status code `407` when making request `GET <...>`.

Running pkl project resolve --http-proxy http://myproxy with a .netrc file containing the credentials had the same outcome.

And when trying to run pkl project resolve --http-proxy $HTTP_PROXY i get another error:

Error: Invalid value for "--http-proxy": Malformed proxy URI (expecting `http://<host>[:<port>]`)

Allowing basic authentication in for http proxies would be helpful.