add ProposerIndex() to BlindedBlocks

[GalRogozinski]

Description

Added missing method

[GalRogozinski]

@mcdee will you review?
Thanks in adavnce :-)

[GalRogozinski]

solved conflicts

btw @mcdee, I noticed you added a RandaoReveal() to api/versionedblindedbeaconblock.go but not to spec/versionedbeaconblock.go. Is this on purpose?

[mcdee]

Apologies for the delay on this. I've been considering it, and my concern is that the interface is a subset of a beacon block rather than actually a beacon block. So, for example, there is no ExecutionPayload() method which is something that a user seeing a BeaconBlock would reasonably expect to be able to obtain.

In general it's better to have interfaces with fewer functions, ideally 1. Would it make sense to have interfaces as:

type BeaconBlockSlotProvider interface {
  Slot() (phase0.Slot, error)
}

type BeaconBlockProposerIndexProvider interface {
  ProposerIndex() (phase0.ValidatorIndex,error)
}

...

as that would allow a full range of interfaces that could cover both blinded and unblinded beacon blocks.

[GalRogozinski]

I changed the description and title of the PR accordingly