add autobalancer callback

[holyfuchs]

Add an optional callback for the autobalancer, necessary to recover stuck vaults in FlowYieldVaults in an optimized way.
This was intentionally left out of the constructor to maintain backward compatibility.

[nialexsan]

this would need a redeployment with a new name or to a new address

[zhangchiqing]

if cap.check() == false, would it be possible for it become true again? if not, does it make sense to set self_executionCallback = nil, so that next time the check can be skipped.

[holyfuchs]

This test transaction passes:

// 1. Store resource and issue capability
let box <- Counter.createTestBox()
acct.storage.save(<- box, to: Counter.TestBoxStoragePath)
let cap = acct.capabilities.storage.issue<&Counter.TestBox>(Counter.TestBoxStoragePath)
assert(cap.check(), message: "cap.check() should be true when resource is stored")

// 2. Move resource out
let box2 <- acct.storage.load<@Counter.TestBox>(from: Counter.TestBoxStoragePath)!
assert(!cap.check(), message: "cap.check() should be false when resource was moved out")

// 3. Move resource back
acct.storage.save(<- box2, to: Counter.TestBoxStoragePath)
assert(cap.check(), message: "cap.check() should be true when resource is moved back")

so it is possible to become true again.

[zhangchiqing]

LGTM, some minor suggestions