[new service] The Build Scheduler

[duckfullstop]

AKA service.build.scheduler

The Build Scheduler will be responsible for, well, the scheduling of builds based on listening for triggers.

It'll listen for events emitted by the service described in #1, then schedule these new versions onto the appropriate build queues.

Right now it'll just dispatch them to service.build.docker (for x86_64), but in future this should be expandable to other image types like ACI, as well as multiple arches (like aarch64).

It will watch / listen for builds to complete, and then mark them as completed in its internal data store to avoid building the same thing more than once. There should be an API to clear this cache (either for a certain version or for all versions) and force rebuilds when needed, for example when things like the container OS / dependencies require upgrades, or if a new binary with the same version tag is required (for example, a bad release).