Support stake change actions in the tests

[dnadales]

The property tests do not have actions for changing the stake distributions. We do test with different stake distributions in each test run, but these are constant throughout the trace.

Module Test.Cardano.Ledger.Update.Properties.UpdateSUT contains the actions supported by the SUT:

  data SUTAct UpdateSUT
    = TickAct
    -- ^ Tick for one slot. We do not make the number of slots variable since
    -- this complicates writing trace properties of the update mechanism.
    | UpdateAct (Update.Payload MockSIP MockImpl)

We need to add an action here for changing the stake (e.g. moving stake from one participant to another). After adding this action we will need to change generators and elaborators.