Remove Encoding of values in char** For Labels
- [X] closes #20500

In reviewing this module there is a shared function for object keys and values which encodes objects into a separate buffer and subsequently indexes off of that. Instead of encoding values in a buffer I've updated that function to be a char ** pointing to string representations of the labels (or index / columns, rather). 

This is arguably a pre-cursor to:

  1. #19486 to disentangle date time functions from JSON
  2. #12004 to add indent support (tried this previously but vendoring ujson updates but didn't work because of this limitation)
  3. #27164 because the various formats may need column / index labels at different points in time, so encoding up front makes that very difficult to reuse

The only downside here I haven't been able to figure out is how to deal with date formatting. Right now all labels are written as epochs. I'm sure there is a way to handle but I wasn't clear on what the best way to convert arbitrary input (i.e. object or datetime dtypes) into ISO formats by element where applicable.

cc @jbrockmendel in case you have insight on that