Add update payload to the CHAIN traces

[dnadales]

Currently, the CHAIN traces have no update payload. We need to add this, by modifying the HasTrace instance for CHAIN, and including the update payload.

More module Cardano.Spec.Chain.STS.Rule.Chain function sigGenChain contains an empty update payload:

      bb =
        BlockBody
          delegationPayload
          utxoPayload
          Nothing -- Update proposal
          []      -- Votes on update proposals
          (ProtVer 0 0 0)

we should generate update payload, keeping in mind that we want to do this only sporadically! (we do not expect to have many update proposals in a given epoch).

See also #584

[dnadales]

Closed by #649