Extend eth_getBlockByNumber to support finalized and safe as block tags

[fab-10]

Description

For The Merge the eth_getBlockByNumber must accept 2 new block tags: finalized and safe, as described by the spec:

finalized: The most recent crypto-economically secure block, cannot be re-orged outside of manual intervention driven by community coordination;
safe: The most recent block that is safe from re-orgs under honest majority and certain synchronicity assumptions;