[Bugfix] - Lower case validator in standard models

[montezdesousa]

1. Why?

   • Allow users to send anycase inputs to case insensitive fields, like sort: Literal["asc", "desc"] should support "Asc"

2. What?

   • Create to_lower validator to include standard model fields that can handle this feature
   • Renamed some upper_symbol validators to to_upper for consistency across files and future use cases besides symbol

3. Impact:

   • If there are provider models that don't support lower case fields they will break, so I made sure only included literals or obvious non breaking fields like sort or period

4. Testing Done:

   • Unittests

   • Gradually this should be extended to provider fields, but will keep this contained for this PR