Add remaining major functions to PyDough builtins

[knassre-bodo]

Tasks for this issue:

• Compile a list of all functions that are to be added in PyDough as builtins and include in this issue (e.g. remaining datetime extractors, datetime creation/addition/subtraction, substring operations, getting the current date/time, least/greatest)
• Add PyDough operators for all such functions
• Add SQLGlot transformation bindings for all such functions
• Add e2e or unit tests for all of the above