[Feature] Add mappings endpoint

[cgetzen]

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)