firestore: design doc for custom datatypes

Issues: #1438 #1863 #1740 #1475 #1267 #2643

Past attempts at fixes from the community: #2481 , https://code-review.googlesource.com/c/gocloud/+/50430 (mentions something like [datastore.PropertyLoadSaver](https://pkg.go.dev/cloud.google.com/go/datastore?tab=doc#PropertyLoadSaver) as a potential model for a solution.

We've seen a collection of issues (linked above) related to using user-defined data types with the firestore library. This is definitely a gap and something that's impacted the usability of the library. This issue covers making a design doc for a fix:

* Consolidate and synthesize all the issues raised in the above threads
* Define the criteria for what needs to be accomplished in a fix.
* Propose solution(s) with a basic definition of what the surface will look like.

We'll need to review the design before moving forward to implementation. The design must be not constitute a breaking change.