Feature: JSON schema

[davidlang42]

Add a command to export a JSON schema using https://docs.rs/schemars/latest/schemars/

The idea is that this exported schema will then make it easy to use external json editors to edit roland-rds json files. For example:

• https://github.com/json-editor/json-editor
• https://github.com/RicoSuter/VisualJsonEditor

[davidlang42]

https://github.com/davidlang42/roland-rds/commit/5b31f950ad69fe86d00b8ad0e26034ce340b4c24