Improve the way we combine fields from different forks in Schema Definitions

[tbenr]

Tracking an improvement we could implement, we discussed it here: https://github.com/ConsenSys/teku/pull/6441#pullrequestreview-1180049006

[zilm13]

• include a test for 2 overrides (Capella overrides execution_payload, then Eip4844 overrides execution_payload) if it will be some automated override logic

[lucassaldanha]

@tbenr could you check if we still need this ticket?

[tbenr]

Yes it is still relevant. From the linked comments I can recall that we have on the table 2 options to make it better:

1. my approach: which allows overriding and deletion of previous fork. It is more a "manual" pattern and less integrated as in our "framework"
2. @zilm13's approach which introduces an override option in our "framework" but (if I'm not mistaken) doesn't support deletion (at least in the version reported in the commit)

My version seems pretty straightforward to me, it isn't engineered at all.