JSON Schema support

The schema gets published and updated via https://github.com/go-vela/types where it ends up as a release artifact. The latest schema is available for download from https://github.com/go-vela/types/releases/latest/download/schema.json.

In addition, the schema is available in the JSON Schema Store project at https://www.schemastore.org. Supported editors automatically use schemas that this project provides, including Vela’s schema.