Specify the schema in YAML rather than JSON

[justinvp]

Hello!

• Vote on this issue by adding a 👍 reaction
• If you want to implement this feature, comment to let us know (we'll work with you on design, scheduling, etc.)

Issue details

The Go boilerplate repo already switched over to using a schema defined in YAML rather than JSON. We should do the same here. Just be aware that we also want to do #8, where the provider needs to return the schema as a JSON string, so we'll likely need to convert the YAML to JSON like what's done in the Go boilerplate.