Add Gradle Enterprise Convention Maven Extension

[clayburn]

This PR adds a sample Maven Extension that applies Gradle Enterprise, Common Custom User Data Maven Extension, and a reasonable set of defaults for configuring build scans and build caching. The goal is to provide a sample that makes it simple for organizations to create and host their own, customized version of this extension in order to more easily and consistently apply Gradle Enterprise configurations across an organization.

The commit history here is a mess, but it is almost entirely additions. I see this getting squash merged.

[etiennestuder]

@clayburn I have not gotten to review this, yet, but please make sure all of this runs on Java 8, incl. the build itself. Similarly, the approach of this convention should work on Maven 3.6, 3.7, 3.8, and 3.9. thanks.

[clayburn]

This needs some attention. I'm going to close it but leave the branch around for now until we want to prioritize it again.