Application-specific workflow actions

[Shadowfiend]

Certain applications that use ECDSA keeps (e.g., tBTC) benefit strongly from additional actions beyond the base ECDSA keep actions. As a specific example, tBTC deposits have certain state transitions that are typically performed by users, but that are incentivized to be performed by signers. In these cases, failure to perform the state transition can have consequences on signers, up to and including partial or full bond seizure.

The ECDSA client should have a way to specify, encapsulate, and execute these application-specific behaviors, in a way that can be scaled to support multiple applications and multiple chains.