improvement: improve .dict speed by limiting dict calls

[armandobelardo]

Fixes FER-2799

This PR removes the second call to .dict we'd made in order to get "exclude unsets without defaults", which was to union the dicts from exclude_unset and exclude_none in Pydantic.
For larger models, the second .dict call would take a significant amount of time, and so here during .dict we:

• Get the list of fields set already in the model
• Traverse the models to see if there are fields not in the field set that have non-None defaults
• If there are, add them to the list
• Pass this list as includes within the single pydantic .dict call

[dsinghvi]

Looks like there is a single seed fixture failing on the python sdk?