Gracefully handle unsupported public key algorithms for AKI presence check

[CBonnell]

RFC 5280 requires the presence of the AKI extension in certificates unless the certificate is self-signed.

In the case of self-issued certificates that certify public keys with unsupported algorithms, AuthorityKeyIdentifierPresenceValidator reports an "unhandled_exception" finding as the certified public key cannot be loaded by the Cryptography library. This issue has complicated efforts in linting various PQC certificates (both at IETF hackathons as well as examples included in drafts in LAMPS).

AuthorityKeyIdentifierPresenceValidator should be modified to emit a NOTICE-level finding if it cannot determine whether the certificate is self-signed due to the inclusion of an unsupported public key.