feat: create Action type and send_with_callback

[willemneal]

This adds a new function send_with_callback, which takes a contract with a function link_callback(account_id), and the gas required for the call.

To handle this change I added an Action enum to replace the Balance value type in the map:

pub enum Action {
  Deposit(Balance),
  DepositCallBack(Balance, AccountId, Gas, Balance),
}

Then when responding to claims the it checks the action to determine if a call back is added to the promise chain.

• Add final attached deposit required by callback

[mattlockyer]

I see where you're going with it. Could be really useful to kick off a cross contract call after linkdrop is claimed.

e.g. I deploy this linkdrop proxy, call send_with_callback (restricted to only_owner) then a callback NFT contract and method could mint a free NFT, only for calls with predecessor of the linkdrop proxy contract.

[willemneal]

Bingo! That is exactly my plan.

[mattlockyer]

Yea cause then we don't have to redeploy our linkdrop proxy contracts to do "new things" with linkdrops. We can just keep creating them and changing the send_with_callback args. Great idea! Love the composability!

[willemneal]

@mattlockyer Just realized that we need to add a way to specify how much attached deposit is required for the final callback.

But in other news, I've tested it with my newest project and realized I forgot about the attached deposit when the callback returns.