API endpoint should show "lease" property for cancel lease transactions

[sbrekelmans]

The "lease" property contains the corresponding start lease (type 8) transaction including "amount", "receiver" etc.. The /transactions/info endpoint for type 9 transactions already contains this "lease" property, but other endpoints like /block/at/{height} don't.

[jasny]

All endpoints for transactions should check this.

Note that we might opt to include the recipient and amount for cancel lease v4, so this would only apply to v3 and earlier.