Verify block sig

[shayzluf]

[Part of] #5116

---

Description

In order to keep slasher detect slashing and not store validator keys we need to validate blocks in beacon chain before adding them to block stream.

[nisdas]

This PR uses genesis block root, we need to use genesis validator root

[terencechain]

I don't think this is necessary, the sync will verify proposer's signature as the very first line of defense, there's no reason for RPC streaming to verify proposer's signature again. See:

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ shayzluf
❌ Terence Tsao

---

Terence Tsao seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (v0.11@3a5b86b). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             v0.11    #5137   +/-   ##
========================================
  Coverage         ?   55.25%           
========================================
  Files            ?      283           
  Lines            ?    22569           
  Branches         ?        0           
========================================
  Hits             ?    12470           
  Misses           ?     8337           
  Partials         ?     1762