`SignaturesRule` validation rule

[Mr-Leshiy]

Summary

Add a dedicated validation rule structure for the validating signatures with kid field.
Moving the current fn validate_signatures implementation under the new SignaturesRule type.

Description

• Add a new SignaturesRule type, with the new configuration parameter mutlisig: bool, moving the existing validate_signatures as a SignaturesRule::check implementation.

struct SignaturesRule {
 /// Flag which allows multiple signatures or not
 mutlisig: bool
}

• Add a new validation rule SignaturesRule and add it into the Rules type as a field.