Per-application post-keep-close behavior

[Shadowfiend]

For tBTC, a closed keep may still have BTC in the associated wallet (say, in case of collateralization liquidation). In these cases, the signers may want to do some additional cleanup (e.g., above, the BTC may need to be split up or forwarded to another deposit, depending). These cases are application-specific, but can be quite critical, and in many ways the client side of a keep isn't “closed” until they are completed. Consider how this might slot in.

A minimal version of this could be as simple as allowing for an HTTP request on each client in a signing group that triggers them coordinating a BTC signature and broadcast. Note that this is also a potential integration point for the requestFunderAbort flow.