Support JVM-Config and environment variables for proxy configuration

### Affected version

* v3 series

### Bug description

Despite dozens of community discussions claiming otherwise on Reddit, Stack Overflow, old Apache bug trackers, and new GitHub Apache bug trackers, neither Maven nor Maven Wrapper support `.mvn/jvm.config` nor environment variables for simple proxy configuration.

None of the industry standard mechanisms available to Gradle, to other programming languages, to *Ant*, work in v3 Maven.

* Industry standard, cURL based environment variables: `http_proxy`, `http_proxy`, `no_proxy`
* UPPERCASE environment variables: `HTTP_PROXY`, `HTTPS_PROXY`, `NO_PROXY`
* Maven wrapper `.mvn/jvm.config` files
* Maven's environment variables: `MAVEN_ARGS`, `MAVEN_OPTS`
* JVM standard `JAVA_OPTS` environment variable

As a workaround, `settings.xml` can be hardcoded to apply a proxy configuration. But this makes it significantly harder to override partial Maven configurations, compared with environment variables.

Review the requested .mvn/jvm.config and settings.xml paths alongside the listed proxy, Maven, and JVM environment variables. Inspect linked pull request #11776 before starting, since it is merged. Done means the requested proxy configuration mechanisms and their scope are resolved.

Written by the indexing model from the issue text.