SyncCommitteeMessage production

[Arkenan]

In #800 is_assigned_to_sync_committee is coded, to evaluate, if at a single slot, a validator is part of a sync_committee and should send a message. This issue encompasses:

• Building a SyncCommitteeMessage using get_sync_committee_message(state, block_root, validator_index, privkey).
• The message should be built and logged (mock broadcasting) if the current proposed block was received in the first slot interval or else if the interval has passed (empty slot).

spec ref: link