SOLR-18152: improve body format OpenAPI annotations to fully communicate SchemaDesigner API

[epugh]

https://issues.apache.org/jira/browse/SOLR-18152

Description

During integration of SchemaDesigner V2 APIS into Admin UI, it was flagged that we don't properly document the addField / add-field commands.

Before today's fix, the generated JS/SolrJ clients would have sent {"addField": {...}} (camelCase) , but the server doesn't consume that, it consumes add-field.

Solution

Now we do:

curl -X POST "http://localhost:8983/api/schema-designer/mySchema?schemaVersion=3" \
  -H "Content-Type: application/json" \
  -d '{
        "add-field": {
          "name": "keywords",
          "type": "string",
          "stored": true,
          "indexed": true
        }
      }'

This applies to add-dynamic-field, add-copy-field, add-field-type as well!

Tests

Manual and unit