Generic pattern for serializing dictionaries in serdes

[schrockn]

Summary & Motivation

Serdes does not handle cases where there are non-scalar, but serializable keys. @dpeng817's previous solution involved implementing a custom serializing, which seemed complicated and magical.

This was removed in #24729 in lieu of encoded key value pairs as a record itself, which was much more obvious and involved less code.

This PR makes that more generic, so we could do it for any dictionary.

If we like this we can move it into serdes proper.

How I Tested These Changes

BK

Changelog

NOCHANGELOG