Adds `expansionRate` field to the `StorageRequest` object, to mirror the object in the contracts.
The CodexClient api module uses a default value of 100, which means that all SPs will be available to reserve slots immediately, when the functionality is finally implemented. It was chosen to provide as little disruption to existing as possible, and may change in the future.
The default value for the REST API endpoint was 60, meaning 60% of the total time before expiry will have all addresses eligible to fill the slot.
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