REF: collect validator methods for DTA/TDA/PA
Zero logic changes in this PR.

xref #33685

We have a _lot_ of methods for converting arguments to DTA/TDA/PA/DTI/TDI/PI methods to/from i8 representations.  ATM these have slightly different behavior, which we should try to bring into sync (and then de-duplicate the code).

This PR just collects all those methods in one place to make the upcoming steps easier.

Two more methods that I think/hope can eventually be added to this grouping are the comparisons and get_loc, but those are more complicated, but those are more complicated, so holding off on those for now.