nGQL: add new datetime functions like datediff and dateadd

[Sophie-Xie]

Describe the background of the requirement
It's quite a common requirement from customers to calculate the datetime type, e.g, and today it's only possible to do so via timestamp operations, not convenient at all;

Describe the detail of the requirement
Propose to add some new datetime functions, e.g.:

datediff(date1, date2) to return the days between date1 and date2;
dateadd(date1, n) to return a new date value of date1+ n days, n can be a negative number.
etc.

Describe the scenarios of the requirement
This can be helpful in various scenarios when users need to calculate against datetime data type.

Describe the values of the requirement
General: nGQL usability
Values for customer: simplify writing nGQL when datetime calculations are involved.
Importance of the business project: it's common request.

from jingchun