Statically verify ruma routes with the spec's openapi docs

[ShadowJonathan]

Mainly for methods and (maybe) paths, as https://github.com/ruma/ruma/commit/a39bee994985ae2ba05c317edac4a32764aa5492 kinda shows, it could help somewhat with verifying that everything is a-okay.

[jplatte]

I realized yesterday that a thing we built at work end of last year could help with this: https://github.com/svix/openapi-codegen (could use it to generate tests, either ones that verify properties of the source code, or actually ones that call into Ruma potentially).

Could also maybe be used to generate the basic structure of endpoint modules (but not the full thing, at least not before some major additions in machine-readable annotations to the spec).