Add Audit of BlockHeader by number

[kdeme]

With the addition of BlockHeader by number in the Portal network (ethereum/portal-network-specs#334) we should be able to see its availability on the network in glados just as is done for BlockHeader by hash, block bodies and receipts.

Tasks

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1. block number lookup in 4444s audit strategy
2. block number lookup in Latest audit strategy
3. block number lookup in Random audit strategy

[carver]

This has several subcomponents that could be split into different issues to work on. For example, add block number lookups to the following audit strategies:

• 4444s
• latest
• random