class AsyncSigningHandle

A handle for an asynchronous signing operation.

When `PrivateKeySigner::Sign` is implemented asynchronously, it returns an

instance of a concrete implementation of this class. This handle is used

to manage the asynchronous signing operation and can be used to cancel the

operation via `PrivateKeySigner::Cancel`.

Users must provide their own concrete implementation of this class. The

handle can store any state needed for the asynchronous operation.