Allow Basic Authentication in http_proxy

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://[:]`)
```

Allowing basic authentication in for http proxies would be helpful.

Start with the `pkl project resolve` entry point and the `--http-proxy` handling, then reproduce the 407 response using `HTTP_PROXY` and a proxy URI with credentials. Done means authenticated HTTP proxies are accepted and requests succeed with credentials supplied in the proxy URI; the issue also notes that `.netrc` currently has no effect.

Written by the indexing model from the issue text.