Update example scripts and make peer_creds unconditionally callable

[TimothyClaeys]

• FIx the broken example scripts
• Use real X.509 certificates in the examples (authentication keys are Ed25519) --> remove pickled dictionary with legacy credential store
• remote_cred_cb takes a Callable that returns the remote credentials
• the return value of remote_cred_cb should go through _parse_credentials to parse/verify the RPK/certificate and extract the remote public authentication key
• Add a custom CBOR encoder to properly encode COSE header maps with COSE header attributes
• remove dependency on asn1crypto (everything can be done with the X509 module of the cryptography package)