API: Design for API for Assets and Asset Groups

### Overview
As a BOP or Toolkit user, I want to be able to view Assets and Asset Groups

### Action Item as
- [ ] Document API specs for Asset and Asset Groups
- [ ] Based on the spec, break down into additional issues
- [ ] Spec includes separate APIs for Assets and Asset Groups
- [ ] Spec includes ability to query by all fields, unless determined not necessary

Start by reviewing the repository's existing API conventions and the current Asset and Asset Group models or entry points. Document separate APIs, query behavior for available fields, and any decisions about fields that should not be queryable. Break the resulting specification into follow-up issues and confirm every checklist item is addressed.

索引モデルが issue の本文から書いたものです。