Move json::to_string response processing to http thread pool

[heifner]

Change Description

• Resolves Add json::to_string to http thread pool #6891
• Move the conversion from fc::variant to JSON string reponse into the http thread pool to reduce the amount of work done on the application thread.

Consensus Changes

API Changes

• The http_plugin url_response_callback changed from std::function<void(int,std::string)> to std::function<void(int,fc::variant)>
  -- Code calling the url_response_callback is now expected to provide a fc::variant instead of a string. This means the the type must be setup with FC_REFLECT so it can be converted to variant. This was already the case for all plugins in eos repo, but if external plugins were doing their own json conversion then that will no longer work.

Documentation Additions