EIP-6475: Add SSZ Optional[T] type (and drop selector byte in Union None case)

[etan-status]

Add specification for EIP-6475 support for SSZ.
Remerkleable impl: https://eips.ethereum.org/assets/eip-6475/optional.py
Tests: https://eips.ethereum.org/assets/eip-6475/tests.py

We could possibly change all planned usage of Union with Optional, and introduce the conceptually more complex Union once needed. Optional serialization can be more optimized than Union.

Discussion: https://ethereum-magicians.org/t/eip-6475-ssz-optional/12891

This PR builds on prior work from:

• @zah at Consider possible improvements to the SSZ spec before phase0 is launched #1916

[hwwhww]

I would like to update all Python Optional to PyOptional in this PR to avoid confusions.

@etan-status Thanks for adding remerkleable implmentation. Do you want to open a PR in remerkleable too?

[etan-status]

Sure, I can open a PR against Remerkleable as well, once the specs are accepted.

With the Union also dropping the selector byte for the None case, it actually becomes a superset of Optional, so the implementation is a bit easier than in the EIP (which targets the current Remerkleable).

[etan-status]

StableContainer (EIP-7495) supports Optional in an even more compact format, and is also forward-compatible; if containers are extended later, proofs for known fields can still be processed even if the verifier code is not updated to support later added fields.