client-sdk: Add PrettyPrint to StakeThresholds

[matevz]

This PR implements PrettyPrint() for StakeThresholds. Improves output of oasisprotocol/cli#318 to

$ oasis paratime show parameters --network testnet
Network:        testnet
ParaTime:       sapphire

=== ROFL PARAMETERS ===
  Stake thresholds:
    App create: 10000.0 TEST

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[kostko]

=== ROFL_STAKETHRESHOLDS PARAMETERS ===

This looks ugly, why is it not rofl.StakeThresholds PARAMETERS or similar?

[matevz]

=== ROFL_STAKETHRESHOLDS PARAMETERS ===

This looks ugly, why is it not rofl.StakeThresholds PARAMETERS or similar?

The idea was to resemble oasis net show parameters output, e.g.

=== SCHEDULER PARAMETERS ===
  min_validators: 3
  max_validators: 110
  max_validators_per_entity: 1
  reward_factor_epoch_election_any: 0

=== BEACON PARAMETERS ===
  backend: vrf
  vrf_parameters:
    alpha_hq_threshold: 3
    interval: 600
    proof_delay: 300
    gas_costs:
      vrf_prove: 1000

=== GOVERNANCE PARAMETERS ===
  gas_costs:
    cast_vote: 1000
    submit_proposal: 1000
  min_proposal_deposit: 10000000000000
  voting_period: 12
  stake_threshold: 68
  upgrade_min_epoch_diff: 24
  upgrade_cancel_min_epoch_diff: 15
  enable_change_parameters_proposal: true
  allow_vote_without_entity: true
  allow_proposal_metadata: true

But I agree, it's not readable. This should look better:

=== ROFL PARAMETERS ===
  Stake thresholds:
    App create: 10000.0 TEST
```