add a proxy() setter with credential support

[jaemk]

Adds proxy(url) on every Update / ReleaseList builder and on Download, for the authenticated
proxy case HTTP_PROXY / HTTPS_PROXY cannot cover. Completes CORP-3 in
specs/corporate-network-config.md.

• Add proxy(url) to request_config_setters! and to Download. Credentials may be embedded in
  the URL (http://user:pass@proxy.corp:8080) and are sent as Proxy-Authorization. HTTP CONNECT
  only.
• Collapse the cert-only client builders into a single ClientConfig { certs, proxy } feeding
  build_configured_client / build_configured_async_client, so add_root_certificate and
  proxy are applied to the same crate-built client and either one alone triggers the build. An
  injected client still wins, per slot.
• Add Error::InvalidProxy { source }, deferred from the infallible setter to build() /
  download_to like the header and certificate errors. check() precedence is header, then
  certificate, then proxy; a generic build failure with no certificates configured is reported as
  a proxy failure rather than blaming a certificate the caller never set.
• Redact the proxy password from the error text (including the wrapped client error, which may
  quote the URL) and from the hand-written Debug of RequestConfig and Download.
• Forward the proxy to the download in build_download, so the release listing and the asset
  fetch take the same route.
• reqwest applies the proxy alongside the env vars (first match wins); ureq has a single proxy
  slot, so the configured proxy replaces the env-var one.

Tests drive a real download through a loopback proxy stub at an unresolvable .invalid target
host, covering both reqwest's absolute-form GET and ureq's CONNECT tunnel, plus unit coverage for
the error attribution, the redaction, the Download forwarding, and the builder surface.

make ci is green.