json: Add direct way to query a config option
json: Add direct way to query a config option

* src/gpgme-json.c (op_config_opt, hlp_config_opt): New operation.
(process_request, hlp_help): Add it.

--
This is more conveniant API for most query operations where
a single option is required.

An example would be:
{
    "op": "config_opt",
    "component": "gpg",
    "option": "default-key"
}

Which results in:
{
   "option":       {
           "name": "default-key",
           "description":  "use NAME as default secret key",
           "argname":      "NAME",
           "flags":        0,
           "level":        0,
           "type": 1,
           "alt_type":     1,
           "value":        [{
                           "string":       "F462B6B1",
                           "is_none":      false
                   }]
   }
}

gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme