[RPC-Spec-V2] chainHead/follow: Provide multiple block hashes to the initialized event

[lexnv]

Backport:

• chainHead/follow: Provide multiple finalized block hashes by the Initialized event json-rpc-interface-spec#136

The initialized event of the chainHead_follow subscription must provide multiple finalized block hashes.

This information allows clients that are disconnected, and that want to reconnect, to not lose information about the state of the chain.