Added mining for POS

[Giulio2002]

No description provided.

[Giulio2002]

The Proposing process works as following:

• We start the PoS proposer by using the --proposer flag, unlike --mine it does not require etherbase to be set
• We have a thread which at intervals updates the payloads using the transaction pool in ethbackend.go
• if etherbase is set, we set block's fee recipient to it, if it is not set, then we use the suggestedFeeRecipient
• There is some clocking between the ethbackend thread and the proposer thread, the ethbackend starts a timer when getPayloadV1 is invoked to make sure that we gave some time for the proposer thread to update/compute the block. we can do 1 block/second with no problem which is Ethereum's limit anyway (--time-per-slot=1s in mergemock).
• The value of the proposer flag is also passed to ethbackend to prevent access to GetPayloadV1 and ForkChoiceUpdatedV1 (assemble case only) with a proper error, i decided not to start the goroutine in the constructor of the backend server but in backend.go, just personal taste but it can be changed.

[yperbasis]

You don't need a separate mutex for condPauseAssemble: just re-use mu.

[Giulio2002]

We cannot do that as reusing mu, would cause mu.Lock be called multiple times without any unlocks aka. stalling everything. the reason of this is that what Wait essentially does is locking the mutex, so we cannot reuse the same mutex. same goes with Broadcast, which essentially unlocks the mutex.

[Giulio2002]

note that we pass in a pointer, not just a value

[yperbasis]

I've double checked https://pkg.go.dev/sync#Cond.Wait and the advise is to check for a condition in a loop. So we can use a single sync.Cond (and its mutex) for everything, just with different checks. I've submitted 4de1c30 to that effect.