DDEV-get add-ons: Build one yourself, maintain one for the community

DDEV has had add-ons in recent versions. These are separately maintained integrations with technologies like elasticsearch, cron, solr, etc. Almost any DDEV feature can be rolled in an add-on. Official add-ons have automated tests and a maintainer.

We'll look at what's already there (lots of them, `ddev get --list --all`!) and how to contribute to them, maintain them, and create one from scratch, even to scratch the itch of just your team.