Remove direct use of PgpSignatory in the Sign taskConceptually, the signing plugin allows for using multiple signatoriesthat implement different signing methods. However, in practice the Signtask always casts the signatory instance returned by Sign.getSignatory()into a PgpSignatory. This makes it impossible to use a signatory that isnot derived from PgpSignatory.The Sign task needs to access the PgpSignatory in order to retrieve thekey id which serves as task input value for the signatory property.This commit fixes this direct dependency by adding the methodgetInputValue() to the Signatory interface. This method returns a valuerepresenting the input value of the signatory property. The Sign taskcan then use this method for retrieving a value that represents thesignatory property for a specific signatory implementation.