NFC: inline insert/delete update handling

[RReverser]

Description of Changes

Instead of a separate TableOp enum, and 4 fields to handle insert/delete metadata in DbOp, use a nullable struct that contains non-nullable object + bytes pair internally:

• if insert is present (non-nullable) and delete is nullable, that naturally indicates insert operation
• if insert is nullable and delete is not, that's a delete
• if both are non-nullable, it's an update
• if both are null, it's an internal "no change" state

This simplifies and shortens update handling as well as reduces risk of state getting out of sync - e.g. TableOp saying that the operation is insert should exist but field containing inserted value being null. Now nullable struct itself communicates whether there is an insert or not, without a separate enum.

API

• This is an API breaking change to the SDK

If the API is breaking, please state below what will break

Requires SpacetimeDB PRs

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[jdetter]

LGTM - seems like a generally good simplification 👍