fix: replace oneOf with anyOf

[jeluard]

oneOf is currently used the following way (except for enums):

• first element is an object from phase0
• then a couple of variations per fork, adding extra properties

So when an object from a later phase (e.g. bellatrix) has to be validated, it also matches the definition fromphase0, .. (unless it's more complex than just new properties).
Which is incorrect as oneOf must match a single definition.

What we want here is anyOf, that matches if one or more definitions matches.