built-in-function: Support char(int) function

[jingchen2222]

Is your feature request related to a problem? Please describe.

char(expr)
Returns the ASCII character having the binary equivalent to expr. If n is larger than 256 the result is equivalent to chr(n % 256)

Examples:

> SELECT char(65);
 A

Reference:

https://spark.apache.org/docs/2.3.0/api/sql/#char

Describe the solution you'd like

We do provide a built-in function development guide for you.
Please check OpenMLDB Build-In Function Develop Guide or the Chinese version for help.

Please make sure you:

• Add built-in C++ function in src/udf/udf.h and src/udf/udf.cc
• Register function to default library from function void DefaultUdfLibrary::IniStringUdf() in src/udf/default_udf_library.cc.
• Add related unit test in src/codegen/udf_ir_builder_test.cc
• Documenting function

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[Ivyee17]

I want to help this, plz assign it to me. WeOpen Star. @lumianph