Updated validator spec with rules for including execution requests in the beacon block body

[lucassaldanha]

• Defined/Updated ssz_serialize and ssz_deserialize functions
• Updated GetPayloadResponse with execution_requests: Sequence[bytes]
• Updated get_payload definition with extra detail re execution_requests
• New section in "Constructing the BeaconBlockBody for ExecutionRequests"
• Updated list[bytes] to Sequence[bytes]

[lucassaldanha]

cc @mkalinin

[lucassaldanha]

@hwwhww could you help me understand the test failures? 😢

[jtraglia]

@hwwhww could you help me understand the test failures? 😢

I think I know the reason. List is a remerkleable type and requires a max length component. For example:

List[DepositRequest, MAX_DEPOSIT_REQUESTS_PER_PAYLOAD]

And in this PR we use List without a length in the updated GetPayloadResponse type. Instead, we can use Sequence which does not require a max length component. I'll push a commit for this.

[hwwhww]

it seems @jtraglia has solved the linter issue? (🙏)

[hwwhww]

LGTM!

Thank you @lucassaldanha!