Add response_field dsl

It's just add example metadata to help to describe response field(Fix https://github.com/zipmark/rspec_api_documentation/issues/56).
Raddocs, or any other viewers can use metadata.
I'm going to propose a pull request to add response field table to raddocs, if this pull request is merged.
It would be better if the DSL validates real response data, but I have no good idea yet :expressionless: