Implement the basic REST API for multimap

1. Retrieves the values of a key in the multimap

GET /rest/v2/multimaps/{multimapName}/{multimapKey}

2. Adds a value on a multimap. The body contains the value

POST /rest/v2/multimaps/{multimapName}/{multimapKey}

3. Deletes a Key and Value of a multimap. The body contains the value

DELETE /rest/v2/multimaps/{multimapName}/{multimapKey}