Add specs for vanity art

[etan-status]

Update specifications to match client implementation behaviour regarding optional vanity art being displayed when certain conditions are met.

[ppopth]

Why should we have all the clients follow this art spec?

This seems to me that it's an implementation thing rather than a spec thing. The protocol will work fine if not all the clients follow the art spec.

Even though it's marked as optional, it's still a sign that we convince all the clients to follow the spec if we put it in the spec.

[ppopth]

This seems related to #2875

[etan-status]

It's all optional and "may". This PR documents existing client behaviour. The panda is in most of them, the owl in some, blobfish hasn't hit yet.

• Nimbus: https://github.com/status-im/nimbus-eth2/tree/unstable/beacon_chain/consensus_object_pools/vanity_logs
• Lodestar: https://github.com/ChainSafe/lodestar/tree/unstable/packages/beacon-node/src/chain/blocks/utils
• Lighthouse: https://github.com/sigp/lighthouse/blob/21dec6f603edd53ad8d2372941c77ab8098d3b5a/beacon_node/beacon_chain/src/block_verification.rs#L90-L112
• Prysm: https://github.com/prysmaticlabs/prysm/blob/a7c9c76b18fb71cb680c81b69414e11c97db7ca2/beacon-chain/blockchain/merge_ascii_art.go
• Teku: https://github.com/ajsutton/teku/blob/e77cd2a36ee1c341dd367620c0025c2bce0be4de/ethereum/statetransition/src/main/resources/tech/pegasys/teku/statetransition/forkchoice/panda.txt