Sign task

This is a signing task that signs the contents of debusine:signing-input artifacts on a signing worker.

The task_data for this task may contain the following keys:

purpose (required): the purpose of the key to sign with: uefi or openpgp (needed separately from key so that the scheduler can check whether the worker has the necessary tools available)
unsigned (Multiple lookup, required): the debusine:signing-input artifacts whose contents should be signed
key (string, required): the fingerprint of a debusine:signing-key asset to sign with; must match purpose

The output will be provided as debusine:signing-output artifacts, each of which is related to the corresponding debusine:signing-input artifact.