[Feature] Add mappings endpoint

curl approach to see the fields: 

```
curl localhost:9200/_mappings | jq '.drawing_revisions_1_2_1.mappings.drawing_revisions.properties | keys'
```

Input: name of index
Output: names of attributes

- Is displaying the "type" of the attribute important?
- Should the command return all fields of all indices, or a single index? (how close do we want to match the API)