Add command to allow swaps via the Broker

[bayological]

Description
With the launch of MU02, the capability to perform swaps via a command line interface has been discontinued. We need to reintroduce the functionality previously provided by the now obsolete 'exchange' command of the celo-cli. To address this, we will create a new mento-cli that will allow users to interact with the mento protocol. At this stage the cli should have a single command to allow users to swap any stable asset for a collateral asset using the new Broker contract.

Acceptance Criteria

[bayological]

Story Points: 8
See the discussion in Sprint Poker #​​33

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