Add validation for paths in config

[wjt]

Previously, if a filename referred to in the configuration does not exist, that would only be discovered when the hook which uses that key runs, which may be minutes or hours into the build.

Add a way to specify in the schema that certain keys must be set to a single path to an extant file, or a list of paths to extant files.

Add key_type = path(s) annotations to the schema for all (I think) of the file-like keys used by hooks in this repo.

https://phabricator.endlessm.com/T35517

[wjt]

I should be clear that this is only very lightly tested. I aimed to spend no more time on it than the image builder spent on a doomed image build overnight :)

[wjt]

So lightly tested that it fails its tests!

[wjt]

Oh I guess I can't use match unless we bump our minimum Python version to 3.10 (i.e. Endless OS 6 is required).

@dbnicholson I'm curious what you think about this approach, before I mess with going back to if/elif etc.

Of course this wouldn't catch the error at PR time – I think the way to do that would be to have a list of known combinations of fields and have a GitHub Actions job which validates the config for all those. (And a similar job in the private config repo which is where the error occurred.) My old idea there is to again abuse the schema. Add the following to the config:

[build]
configtuple = ${product}-${arch}-${platform}-${personality}

and then add to the schema:

[build]
configtuple_values = eos-amd64-amd64-base
                     eos-amd64-amd64-en
                     # ... etc