Missing required validation for some attributes of block headers in PoM transaction

[nazarhussain]

Expected behavior

All block header attributes are required since these are used in getBlockBytes. So all attributes must be present and required in schema validation.

Actual behavior

There are currently height, maxHeightPrevoted, maxHeightPreviouslyForged, previousBlockId is not listed as required attributes.

https://github.com/LiskHQ/lisk-sdk/blob/62b4c1485e8010e4fdfe96c7e42cfce23d8c5cc2/elements/lisk-transactions/src/15_proof_of_misbehavior_transaction.ts#L38-L50

Which version(s) does this affect? (Environment, OS, etc...)

4.0