[1.9.x] Honor system proxy credentials in HTTP transport

[goutamadwant]

Fixes #2139.

Backport the system proxy credential fix to Resolver 1.9.x, the lineage used by the reported Maven 3.9.16 installation. With aether.connector.http.useSystemProperties=true, matching http.proxyUser/http.proxyPassword or https.proxyUser/https.proxyPassword can authenticate the configured proxy when explicit Resolver authentication is unavailable.

Keep fallback credentials separate from repository authentication and clear them when redirects change proxy or become direct. The default configuration and public API remain unchanged. Tests use the maintenance branch's JUnit 4 and Jetty 9 harness, with a test-only Jetty proxy dependency for HTTPS CONNECT.

The native regression fails with HTTP 407 on unchanged 1.9.x. Validation:

• mvn verify: all 18 modules passed; 1,053 tests, zero failures/errors, one existing skip.
• mvn -Prun-its verify: passed, including 36 integration tests and one Invoker build.
• Full HTTP transport suite additionally passed 85 tests on Java 8, with no failures/errors/skips, covering transfers, CONNECT, precedence, credential isolation, and redirects.

Full reactor builds used Java 21 and Maven 3.9.16; the additional HTTP test run used Java 8u504.

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• Regression tests fail without the runtime fix.
• mvn verify passed.
• mvn -Prun-its verify passed.
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