Key/Backend paring config

[jevonearth]

Problem Statment

If a signatory config has multiple backends setup, that contains the secret for an address, it is not possible to specify which backend or backends should be used, and (if more than one) in what order.

User Story:

As a signatory operator with multiple vaults configured in my signatory.yaml file, I want to set preference on an address in the signatory config so that it will only use specified backends for signing.

If I have an address such as tz3MhmeqpudUqEX8PYTbNDF3CVcnnjNQoo8N and I know that the private key for this address is only in backend yubi_abc I want to "pin" my configuration so that signatory only uses that backend and no others.