Adds the ability to rotate discovery keys on running clients, needed for
testing upcoming disco key distribution changes.
Introduces key.DiscoKey, an atomic container for a disco private key,
public key, and the public key's ShortString, replacing the prior
separate atomic fields.
magicsock.Conn has a new RotateDiscoKey method, and access to this is
provided via localapi and a CLI debug command.
Note that this implementation is primarily for testing as it stands, and
regular use should likely introduce an additional mechanism that allows
the old key to be used for some time, to provide a seamless key rotation
rather than one that invalidates all sessions.
Updates tailscale/corp#34037
Signed-off-by: James Tucker <james@tailscale.com>
James Tucker