feat: support category attribute when fetching all plugins

### Description

The new APISIX Dashboard does not aim to maintain a static set of plugin configurations. Instead, it will be more efficient for the Admin API to provide all the lists and schemas for the plugins directly.

Endpoint: `/apisix/admin/plugins?all=true` returns all schema

## TODO
- [ ] Support a `category` attribute in the plugin schema

Start by tracing the Admin API endpoint `/apisix/admin/plugins?all=true`, which returns all plugin schemas. Read the code that assembles the plugin schema response and check how existing schema attributes are represented. Done means the response supports a `category` attribute for plugin schemas returned when `all=true`.

Written by the indexing model from the issue text.