Move atomic hash from Block to Header

[AskAlexSharov]

Reason 1: NewEVMBlockContext -> engine.Author(header) -> *Bor ->

hash := header.Hash()
if address, known := sigcache.Get(hash); known {
   return address, nil
}

it's hard and not so useful to pass block through this abstractions.

So, RPC does hash every requested header here.

Reason 2: In E3: many RPC will read 1 transaction only. So, likely we will have many places where no Block object.

[AskAlexSharov]

@yperbasis what do you think about:
https://github.com/erigontech/erigon/pull/11513/files#diff-e177ddb5610d1f3426c15cb512999613785b20838954e48187883e6263bb282bR118

type.Block immutability was enforced by geth team. Maybe it's not so bad idea to enforce Header immutability also.
Only place where we need mutable header is: mining engine.Prepare/engine.Seal

There is also Finalize/FinalizeAndAssemble mutating Header: but I think Finalize must not mutate header - because Finalize called in execution and FinalizeAndAssemble called in mining. Changed Finalize.

[yperbasis]

Let's remove the same line from (c *Clique) Finalize

[yperbasis]

AuRa & Merge consensus don't call block.WithSeal when building blocks. We could change them to ensure they do. But I think a better option is to set b.header.mutable to false at the end of NewBlock.