chore: automate version / changelog management

[helmturner]

We'll need some form of automated versioning/changelog management, especially considering the compositional nature of the proposed architecture. There are a few options out there, but in my experience Google's Release Please! bot does a nice job. It doesn't over-reach in scope, and it does exactly what it's supposed to do with little setup cost.

It's based on conventional commits, which we implement. Where there are discrepant commit messages, it's possible to override messages where necessary.

The only change this would require to our workflow is to handle squashed commit messages differently, but this can be done either at the top level by repo owners, or preferably, by convention (perhaps even programmatically).

Both the GitHub action and the GitHub app are good options, but whatever route we end up going, I recommend starting by bootstrapping a manifest and config.