Support $__to and $__from variables from UTC

**Why is this needed**: When using a "generic" datasource like the infinity plugin or json etc, often then restful endpoints require times be in UTC, but for which a user may want to have their dashboard in localtime. This would require something like: ``` ${__from:date:utc:YYYY-MM-DD HH:mm:ss} ``` or maybe ``` ${_to:date:-05:00:YYYY-MM-DD HH:mm:ss} ``` **What would you like to be added**: This way, a user could create a request using dashboard variables in a timezone different from what their dashboard is in. **Who is this feature for?** Well, it came from a project from IBM, but they have a workaround.