Handle GPG revocation signatures

[lukpueh]

Description of issue or feature request:
#257 adds gpg self-signature verification support, for signatures types 0x10-0x13 (certifications) and 0x18 (subkey binding signature). The added infrastructure may be used to also consider other signature types such as,

• 0x20: Key revocation signature
• 0x28: Subkey revocation signature
• 0x30: Certification revocation signature

See RFC4880 5.2.1. Signature Types and 12.1. Key Structures for details about the signatures types and where in the key bundle they occur.

Current behavior:
Revocation type gpg signatures are ignored.

Expected behavior:
Handle revocation type gpg signatures, e.g. ignore revoked keys or certificates and/or warn user about them.

[lukpueh]

Since we are in the process of moving our gpg implementation to securesystemslib, we might want to address the issue over there (see secure-systems-lab/securesystemslib#163).

[lukpueh]

Will be fixed with secure-systems-lab/securesystemslib#190 (also see #275)