cmd/network/governance: Parameter change proposal support

[ptrus]

Adds support for submitting a parameter change proposal (requires oasisprotocol/oasis-core#5472 for pretty printing transaction body to work)

$ cat desc.json
{
        "upgrade_cancel_min_epoch_diff": 15
}

$ oasis network governance create-proposal parameter-change governance ./desc.json
You are about to sign the following transaction:
Method: governance.SubmitProposal
Body:
  Change Parameters:
    Module: governance
    Changes:
      - Parameter: upgrade_cancel_min_epoch_diff
        Value: 15
Nonce:  61
Fee:
  Amount: 0.0 TEST
  Gas limit: 1321
  (gas price: 0.0 TEST per gas unit)