Introduce GITHUB_PROXY environment variable support in http-client

Describe the enhancement
For complex setup with self-hosted Github Action runners were all requests toward Github hosted servers needs to go thru an intercepting webproxy please introduce support for aenvironment variable like GITHUB_PROXY or GITHUB_HTTPS_PROXY.

When the http-client detects an access towards a server hosted by GitHub, use the proxy server defined in GITHUB_PROXY.

Currently self-hosted github action runners need to set HTTPS_PROXY and NO_PROXY, which is hard to get right in a strict setups, as the semantic is wrong: it's deny listing (in NO_PROXY) instead of allow listing only certain target hosts.

By introducing a special GITHUB_PROXY or GITHUB_HTTPS_PROXY variable all traffic toward GITHUB could be forced thru intercepting web proxy, but no other traffic would be routed thru any proxy, as HTTPS_PROXY, HTTP_PROXY and NO_PROXY are widely used in different syntaxes by all kind of software.

Start by locating the http-client entry point and its existing handling of HTTPS_PROXY, HTTP_PROXY, and NO_PROXY. Determine how requests to GitHub-hosted servers are identified, then add support for the requested GitHub-specific proxy environment variable without routing unrelated traffic through it; verify the behavior with the project's existing tests.