Add case-insensitive configuration

[knassre-bodo]

Goal: add a new case_insensitive configuration to PyDough (default=False). When True, all name matching for PyDough collections/properties in the metadata is case-insensitive. This means that accessing Customers vs CUSTOMERS vs customers vs cUsToMeRs is all the same object in PyDough.

This can be implemented by having the following occur when the config is True:

• All terms placed in any terms sets/dictionaries in the QDAG nodes are all changed to lowercase (confirm no conflicts)
• All calls to get_term/get_expr /get_collection change the input string to be lowercase
• Side effect: column output names are always lowercase, unless columns argument of to_sql or to_df is used to rename them

This feature must be extensively documented in the usage file.