[SQL] Support MySQL PI and TRUNCATE function.

[dongzl]

What would you like to be added:

Why is this needed:

ref #454

official document:
https://dev.mysql.com/doc/refman/5.6/en/mathematical-functions.html#function_pi
https://dev.mysql.com/doc/refman/5.6/en/mathematical-functions.html#function_truncate

reference code:

https://github.com/arana-db/arana/tree/master/pkg/runtime/function2/abs.go