[MNG-7918] Allow to register maven core extension xml in MAVEN_HOME

**[Brian Demers](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=bdemers)** opened **[MNG-7918](https://issues.apache.org/jira/browse/MNG-7918?redirect=false)** and commented

Related to MNG-7772

I think it would be useful to define a extensions.xml in `${MAVEN_HOME}/conf`
This would allow organizations using a custom Maven dist (e.g. via the Maven Wrapper) to easily configure/manage dependencies, without needing to bundle jar files in the dist.

This same Maven dist could then be used across multiple projects in an organization, and _should_ resolve some of these types of [concerns](https://issues.apache.org/jira/browse/MNG-7772?focusedCommentId=17769736&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-17769736)

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No further details from [MNG-7918](https://issues.apache.org/jira/browse/MNG-7918?redirect=false)

Start by reading the related MNG-7772 discussion and tracing how Maven core extensions are currently configured and resolved. Define the expected behavior for an extensions.xml file under ${MAVEN_HOME}/conf, including how it interacts with project configuration, and verify the behavior with the relevant Maven extension-loading tests.

Written by the indexing model from the issue text.