Introduces the "sql" backend, and refactors backends behind a nicer
interface (which looks a lot like the one in id/auth). Users can
define arbitrary SQL queries to fetch the key data, to adapt to their
database schema.

Include vendored dependencies for sqlite3/mysql/postgres.

Avoid implementing our own version of asp/userenckey/u2f serialization
and deserialization.

The field is prefixed by an ID in LDAP (separated from the key data by
a ':'), strip it before returning key data from the backend.

This allows Dovecot to fail temporarily if the key lookup backend is
unavailable or has issues talking to LDAP.

The dovecot-keylookupd is a dict-proxy dovecot lookup daemon, which
can read public and private keys from the LDAP backend and use them
for Dovecot userdb / passdb lookups.