boulder: Export a JSON schema for use with editors and schema validation tools

[ermo]

The benefit is that this would enable us to do an automated check of how much of the repo needs to be updated if we change/improve our schema.

In addition, it would provide live feedback to packagers in editors where JSON schemas can be used to provide hints.

We could potentially use https://docs.rs/schemars/latest/schemars/ ?

[jplatte]

I've updated the title to remove "from serde". While schemars interprets serde attributes to give you a matching schema most of the time, this is unrelated to serde itself and only works when Deserialize / Serialize implementations are derived (and certain unsupported / unsupportable attributes are not used, or schemars-specific attributes are added for extra customization).

It should actually be possible to generate a (very limited) schema from an arbitrary Deserialize implementation directly, but that's not what schemars does, and is probably not all that useful for real things due to how limited serde's data model is (it's really not meant for this).

[ermo]

@jplatte If you decide to work on this, feel free to self-assign. =)