Move ExecutionRequests parsing to JSON decoding layer

[arnetheduck]

#7396 moved executionrequest parsing to just after el payload selection - while this is a net improvement, it would be even better if the parsing was done like all other parsing, during web3 request decoding and not as a separate step, after.

This is important because if one EL is broken and ships an invalid execution request, we should select another, in the case that multiple els are connected.