Switch VC <> BN block production call to SSZ (`eth/v3/validator/blocks`)

[jimmygchen]

We currently use the JSON-based eth/v3/validator/blocks (POST) for block production. Lighthouse supports an SSZ equivalent which can reduce serde overhead.

Update the VC to use the SSZ variant for block requests when the BN supports it.

[chong-he]

I am picking this one up, thanks @jimmygchen