User of storage client can listen for events on deals

[hannahhoward]

As a user of the Storage Client module, I should be able to listen for updates on deals being processed, for the purposes of logging, or analysis, or any other reason

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In:

• Add a subscriber / listener function to the Storage Client
• Pass updates to the listener

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Acceptance criteria:

• Subscribe to updates on client
• Propose a deal to a storage provider with a client
• Verify updates come through on listen