Add JSON schema

[krzema12]

To make it easier to build the YAML.
It can be also added here: https://github.com/SchemaStore/schemastore so that it's available by default in IntelliJ-based IDEs.

[Vampire]

Where do you think it makes most sense to host the JSON schema, or what would you prefer?

Basically the two options are to host it in the schemastore repository and thus through their site,
or to host it here and just have an entry in the schemastore catalog that points at the URL where the schema is hosted.

Both have their pros and cons.

For example hosting it here it is closer to the definition and can be developed together with it,
but we probably then here need to build some testing infrastructure.

Hosting it in the schemastore repository, all the infrastructure is already there and you can just throw positive and negative YAML files in the according directories so that the schema is tested against those using Ajv, but it is like having the typing in the catalog instead of with the action itself, besides that the typing specification should most probably change way more seldom than an action changing inputs / outputs.

[Vampire]

Actually, I think I'm sufficiently happy with the schema I created, now I just need to know where I should contribute it to. :-D

[krzema12]

I'd prefer to own it, and keep it in this repo for now (even though we need to add some testing infra), and later on we can mirror it in the schema store or add a reference like you mentioned.