create general string question + string response query type

[tkernell]

create a catch-all string query type

MultipurposeString(string _question) response: string _response

Covers many use cases.

[oraclown]

Nice, thought we already had this, but doesn't look like it in the dataspecs repo. Was implemented as one of the first one's by Mike in telliot here. Suggest naming it StringQuery to match the Python, but it's an easy change on that side otherwise