Emit random seed when E3 is requested

[auryn-macmillan]

When an E3 is requested, a random seed should be emitted.

This seed will be used as a deterministic seed for both sortition and for any common random values that nodes require to generate a shared key for the given encryption scheme.

On EVM environments with access to the beacon chain's randao, we can simply make use of that for now. For other networks, we'll need to find some other credible source of randomness.