Expose network configuration in pex options.

[jsirois]

In particular, these pip options should be plumbed through from the pex CLI & the resolver API:

  -H, --header <key:val>      HTTP header to include in all requests. This option can be used multiple times. Conflicts with --extra-index-url.
  --proxy <proxy>             Specify a proxy in the form [user:passwd@]proxy.server:port.
  --retries <retries>         Maximum number of retries each connection should attempt (default 5 times).
  --timeout <sec>             Set the socket timeout (default 15 seconds).
  --trusted-host <hostname>   Mark this host as trusted, even though it does not have valid or any HTTPS.
  --cert <path>               Path to alternate CA bundle.
  --client-cert <path>        Path to SSL client certificate, a single file containing the private key and the certificate in PEM format.

[jsirois]

See #26

[jsirois]

Noting we already handle --trusted-host by implicitly trusting any custom indexes or find links repos specified by the user. This may still leave some corner cases unhandled that use re-directed storage (I think antlr3 did this), but probably fine until a need is demonstrated.