add Revel Digital gadget definition schema

[mtinnes]

This pull request introduces support for the gadget.yaml schema used by Revel Digital gadget definition files. The changes include adding the schema to the catalog, defining the schema itself, and providing an example file for validation.

Schema Integration:

• Added an entry for the gadget.yaml schema in src/api/json/catalog.json, enabling schema validation for files named gadget.yml or gadget.yaml.

Schema Definition:

• Created src/schemas/json/gadget-yaml.json, defining the structure and validation rules for gadget.yaml files. The schema includes properties such as title, author, prefs, and nested definitions for Locale, Pref, Depend, and others.

Example File:

• Added src/test/gadget-yaml/good-example.yaml as a test file demonstrating a valid gadget.yaml configuration, including examples of preferences, locales, and other schema features.

[github-actions]

Thanks for the PR!

This section of the codebase is owned by @madskristensen and @hyperupcall - if they write a comment saying "LGTM" then it will be merged.