[MBUILDCACHE-37] Allow disabled-by-default configuration to be enabled on the CLI

[ahus1]

This allows this plugin to be enabled on the CLI with

-Dmaven.build.cache.enabled=true

when it is disabled by default in the XML.

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[ahus1]

I'm looking forward to have this feature in a patch release as it would allow me to use this extension "opt in" for the developers of the Keycloak project. Thanks!

[ahus1]

I'm happy to see those approvals above. Once this is merged and available as a release (even a pre-release), I'll be able to canary-test this with some users on our project. Let me know how I can help. Thanks!