restgen json access enhancements

[leondz]

currently, when services return json, folk can only specify a top-level dict key from where to get the response

this doesn't work - it might a list, or not top level, or...

proposal: move to a JSONPath in the response_json_field variable

logic:

• if response_json_field contains no JSONPath special chars, treat it as a top-level dict key (retaining backward compat)
• if response_json_field starts with a $, treat is as a raw JSONPath
• if response_json_field contains JSONPath special chars, treat it as a JSONPath, but prepend a $ (this allows python-like structures such as [0][0]
• if we're very nice, write a note suggesting people escape stuff from the YAML parser

JSONPath likes to return a list. hmm

• if the result is a str: return that str
• if the result is a list of 1: return result[0]
• if the result is a list of len>1: return the list
• note that the output should be List[str]. if we're really nice, we could type check that, and log an error if we don't get it back