`swarmctl --swarm https://<swarm> login` runs the OAuth 2.0
authorization-code flow with PKCE against the swarm's authelia as a new
public `swarmctl` client: a pushed authorization request, the browser login
and explicit consent, and the code delivered by form_post to a loopback
port. The access and refresh tokens are cached at mode 0600 under
$XDG_STATE_HOME/swarmctl/ and refreshed under a file lock, since authelia
rotates the refresh token and revokes the login when a used one is
presented again. With --swarm (or SWARMCTL_SWARM), the agent, schedule and
forge verbs send their requests to https://<swarm>/api/ with the bearer
token instead of over the controller's unix socket, which stays the default.
authelia registers the client on every authelia host with a swarm domain:
`kind = "native"`, public, no minted secret, scopes offline_access and
authelia.bearer.authz, audience https://<swarm>/api/ (matched as a URL
prefix), redirect http://127.0.0.1/callback (any loopback port). Every
field is one authelia 4.39.20's validator requires for a public client
holding the bearer scope.
The swarm apex's /api/ answers a refused caller that sent an
Authorization header with the 401 itself instead of the login redirect,
so the CLI can tell "log in again" apart from an answer; a browser still
gets the redirect. The bearer token is not forwarded to the controller.
Refs #4409 (mara: c94764, c94782).
