[Merge] Implement SszUnion structure

[Nashatyrev]

PR Description

Implement SszUnion

Fixed Issue(s)

Part of #4226

Documentation

• I thought about documentation and added the documentation label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

[ajsutton]

LGTM. Nice work - just a couple of small suggestions which probably don't matter for the merge branch but I think we could probably pick this up pretty much as is and merge into master when it's needed.

[ajsutton]

Could/should this just use getSelectorFromSelectorNode(nodeSource.loadLeafNode(selectorHash, GIndexUtil.gIdxRightGIndex(rootGIndex))?

[Nashatyrev]

Tried your suggestion, but nodeSource.loadLeafNode(selectorHash, GIndexUtil.gIdxRightGIndex(rootGIndex)) returns me 0x0100000000000000000000000000000000000000000000000000000000000000 instead of 0x01.
Is it a bug or expected behavior? (the actual value of selector leaf node is 0x01)

[Nashatyrev]

Ok, leaving it 'as is' for now. At least the same code is in the SszListSchemaImpl (borrowed it from there). And probably this needs to be sorted out separately

[ajsutton]

ah yeah of course. We don't store leaf nodes with data shorter than 32 bytes because the hash is just the data padded out to 32 bytes which means they don't have a unique hash. So you have to trim the data back down to the right length after loading. So yeah, the way you have it makes sense (pretty sure I went round this loop doing the list schema code too)...