Autocrypt 2 support

Autocrypt 2 website is at https://autocrypt2.org/
Current specification is at https://datatracker.ietf.org/doc/draft-autocrypt-openpgp-v2-cert/02/

We need to change key generation to generate Autocrypt 2 TSKs, but also need to prepare clients to handle Autocrypt 2 certificates by updating certificate merging and taking key expiration into account when selecting the key for encryption. In the current state we always encrypt to the first subkey, which is a fallback key.