Support the ability to pass rekey to an app call

[robdmoore]

Problem

If you have a smart contract call that needs authorisation to act on your behalf and then rekeys back again it's not currently easily possible to pass rekey in and this extends to the TypeScript client generator output.

Reported via Discord: https://discord.com/channels/491256308461207573/1241689075061231688

Solution

Allow rekey to be passed in to app calls, with appropriate warning about the risk of doing so.