Timing ownership claim attack on new external back-end secrets

A race condition in the secrets management subsystem of Juju versions 3.0.0 through 3.6.18 allows an authenticated unit agent to claim ownership of a newly initialized secret. Between generating a Juju Secret ID and creating the secret's first revision, an attacker authenticated as another unit agent can claim ownership of a known secret. This leads to the attacking unit being able to read the content of the initial secret revision.