Add head producer to beaconmock

[dB2510]

Problem to be solved

While adding support for DutySyncMessage to simnet we found that some VCs use head events stream to get head beacon block root. We need to add support for server side events to beaconmock to serve head events to VCs on simnet.

Proposed solution

• Add a HeadProducer stateful struct in beaconmock.
• It should update head based on a slot ticker (refer: scheduler for a slot ticker).
• When it receives a slot tick, it calculates a new head event and caches it as "current head" in state.
• It also pushes head event to all event subscribers.
• When GET "/eth/v1/beacon/blocks/{block_id}/root" is called it should return "current head" from state.

Out of Scope

None