table: Add TABLE_SET_JSON_FIELD_NAME

table: Add TABLE_SET_JSON_FIELD_NAME

Lets you conveniently set JSON field names in table_add_many. Especially
useful for vertical tables. For example:

table_add_many(t,
               TABLE_FIELD, "Display Name",
               TABLE_STRING, obj->display_name,
               TABLE_SET_JSON_FIELD_NAME, "displayName",
               TABLE_FIELD, "Timestamp",
               TABLE_TIMESTAMP, obj->timestamp,
               TABLE_SET_JSON_FIELD_NAME, "timestampUSec");