Get a payment by ID

(NOTE: This is the same object for "get by ID", and the item that makes up "getAll" elements)

{
    id: PAYMENTID-61b1-420b-9add-c510d44a354f
    source_wallet_id: 43971e26-61b1-420b-9add-c510d44a354f
    target_wallet_id: where applicable,
    requested_amount: {
       amount: amount in pence (e.g. 1000 = £10)
       currency: GBP    
    },
    type: PAYOUT
    status: PENDING
    external_id: your unique reference    
}