Add more "native" support for the oneOf JSON schema

[kdeme]

Issue regarding potentially improving support for the oneOf JSON schema, and perhaps also anyOf and others.

When currently one of two (or more) types are to be accepted, we will typically write something custom, e.g.:

• https://github.com/status-im/nimbus-eth1/blob/master/nimbus/rpc/p2p.nim#L66
• https://github.com/status-im/nimbus-eth1/blob/master/fluffy/rpc/rpc_portal_api.nim#L124

In case this concerns a type that is often used, it can be improved of course by creating a custom type for this and overloading the %, but it still results in something custom for each different group of types.

For some scenarios the Option type can be (ab)used to deal with this, like here: https://github.com/status-im/nim-web3/blob/437c768bc3267da325645a7f346516f75776a05c/web3/ethtypes.nim#L289

But it is not ideal as it:

• will still result in extra null value(s) (in case of return value), or it will not be properly checked for only one of the values (in case of a parameter). (I've checked with AJV schema validator and it does consider the extra null(s) valid according to the schema, but I wouldn't be surprised if some implementations do complain about the null.)
• only works if all types are supposed to be wrapped in an object

PS: Optional fields now can be removed from output and the server also able to handle null fields

[kdeme]

Somewhat related issue as a variant could be used for this: #29