[experiment] Set up per-component subtree splits

OK - I've got this how I want it. Here's the plan:

• We'll use the latest version of the splitsh-lite tool to generate the subtree splits. I've compiled it into a multi-architecture (arm64 and amd64) Docker image based on the work in https://github.com/splitsh/lite/pull/75, with the main difference being that I based the runtime image on alpine:latest so that it would have a minimal shell and be easier to use on CI, without having to do Docker-in-Docker weirdness.
• The splits will be generated by the "subtree split" CI job, upon every commit to the default branch. But only if paths that match drupal_cms*/* have changed.
• The remotes are listed in components.git, which is appended to the .git/config file in the CI environment. Every one of those remotes must correspond to the directory name of a component, and only the ones in that file are going to be pushed to their own repositories. We will push to a remote branch with the same name as the default branch, from which we are generating the subtree splits.

I tested this on my personal GitHub and it seems to have worked beautifully.

This blocks a stable release of Drupal CMS, but I think that the list of components, and their permanent homes, should be finalized during beta, before we tag RC1.

In other words, for beta we can keep using our kludgey zip file. For RC, we want to be distributing our components using Composer, the way they will actually be published out in the world.

This pretty much locks in our distribution strategy, so assigning to @tim.plunkett for final review and approval. I ran it past @catch in Slack and he informally +1ed it as well.

Here we go...

Automatically closed - issue fixed for 2 weeks with no activity.