Second of the `http_proxy` stack. Stacked on #59217 — review/merge that
first; this PR's diff is just the upstream-proxy config.

Adds `UpstreamProxy`: parsing of an upstream HTTP proxy from the
environment (`HTTPS_PROXY` / `HTTP_PROXY` / `ALL_PROXY` and lowercase
forms) with `NO_PROXY` bypass matching delegated to the `proxyvars`
crate, basic-auth credentials kept out of `Debug`/`Display`, and
IPv6/default-port normalization. The proxy server (next PR) uses this to
chain outbound connections through a corporate proxy when one is
configured.

Release Notes:

- N/A