Add an option to enable NTS for an NTP source. Check for NTS-specific
extension fields and pass the packets to the NTS-NTP code in order to
enable the NTS client and server.
If authentication is not enabled in configuration, responses are not
expected to be authenticated. Handle such responses as having failed
authentication.
A case where this could happen is a misconfigured symmetric association
where only one peer has specified the other with a key. Before this
change synchronization would work in one direction and used packets
with an asymmetric length.
MAC longer than 24 octets in NTPv4 packet is supported only for
compatibility with some pre-RFC7822 chrony versions. They didn't use
any extension fields.
Move most of the authentication-specific code to a new file and
introduce authenticator instances in order to support other
authentication mechanisms (e.g. NTS).
