Change of CAST funtion

[廖兰宇 <18...@smail.nju.edu.cn>]

Hi all. 
Function CAST is a built-in UDTF function in IoTDB. We decide to make it a native function like DIFF in IoTDB. We designed CAST according to that of PostgreSQL. There are some changes of CAST function in syntax and cast rules.
An example for how to use CAST function: select cast(s1 as INT32) from root.sg
The data type after AS specifies the target data type you want, all SIX data types of IoTDB are supported.
PR link: https://github.com/apache/iotdb/pull/9226
After merging the PR, the cast rules could be found at: https://iotdb.apache.org/zh/UserGuide/Master/Operators-Functions/Conversion.html


Best,
Liao Lanyu