Add authenticated HTTP proxy support for Bazel downloads

[ptarjan]

This change enables Bazel to work with authenticated HTTP proxies by:

1. Reading proxy credentials from http.proxyUser/http.proxyPassword and https.proxyUser/https.proxyPassword system properties

2. Setting the Proxy-Authorization header directly on HTTP connections to ensure proper authentication with the proxy server

3. Automatically enabling Basic auth for HTTPS tunneling by clearing jdk.http.auth.tunneling.disabledSchemes (which defaults to "Basic")

4. Using Java's Authenticator mechanism for HTTPS CONNECT tunneling, with RequestorType.PROXY check to prevent credential leakage

The ProxyHelper now returns a ProxyInfo object that contains both the Proxy and optional authentication credentials, allowing HttpConnector to set appropriate headers.

Fixes #14675
Fixes #7487
Fixes #6196
Fixes #26674
Related to #601
Related to #587
Related to #11374
Related to #15740
Related to #3993

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[gemini-code-assist]

Code Review

This pull request adds support for authenticated HTTP proxies in Bazel, a significant feature enhancement. The changes involve introducing a ProxyInfo class to handle proxy configurations and credentials, reading credentials from system properties, and using Java's Authenticator for HTTPS tunneling. While the implementation is thorough and includes extensive tests, I've identified a critical race condition concerning the use of the global Authenticator.setDefault. This could lead to authentication failures during parallel downloads. The rest of the implementation appears solid.