Billing request objects' `Status` fields should be typed as enumeration, not `string`

[iankemp-CapitalOnTap]

An enumeration does exist:

BillingRequestStatus

but the BRQ entities that have a Status property define it as a string, not that enum:

• BillingRequest.Status
• BillingRequestWithAction.Status
• BillingRequestListRequest.Status

Other entities are defined correctly e.g. Mandate has MandateStatus, please can BRQ be updated to match.