DM-44635: Nicer GitHub integration config

[fajpunk]

Split out GitHub CI and refresh app config

GitHub CI and refresh app config are now each a separate, all-or-nothing set of config that comes from a mix of a yaml file and env vars.
If a path to one of these GitHub app config files is given in the base config, then:

• All of that particular config is validated and loaded
• Any necessary initialization is done in the lifespan function
• Handlers for that functionality are mounted in the app

Having the whole set of config be required, rather than each config item being optional makes app initialization much less complex.

Config now comes from a mix of environment variables and multiple ConfigMaps (GitHub app integrations and autostart config). It may be worth looking at smooshing it all into one single ConfigMap in the future (or simplifying it some other way), but that would involve a lot of Mobu and Phalanx changes, and I think we can kick it down the road for now.

This is supported by these phalanx changes.

Dynamic GitHub CI app Gafaelfawr scopes

The scopes granted to GitHub CI app jobs now come from config and can be set differently per environment in Phalanx.

[rra]

This looks great.

This path looks familiar from Gafaelfawr: I went down the path of having configuration options giving the path to a YAML file for a while, and it works fine. In the long run, though, I decided to do the work that you suggest in the PR description and unify all the configuration files into a single YAML file that was just a verbatim dump of the config key of the Helm chart, and then only use environment variables for the secrets that don't come from the values.yaml file. I ended up liking that a lot better because it meant there's only one configuration language (well, plus the stuff that has to be environment variables), and I never had to mentally translate between what stuff looked like in Helm and what stuff looked like in the app.

I completely agree that we can kick this can down the road, though.