Add sensors for who authorized a charging session

[rhammen]

Zaptec has no "is authorized" observation: ChargerCurrentUserUuid (722) holds
the token of whoever authorized the current session, and is empty when nothing
is authorized.

• Authorized by — the live token: the RFID card, or "HA/Zaptec App" for a
  remote authorize, which this integration cannot distinguish from one made in
  the Zaptec app.
• Last session authorized by — the same, from
  CompletedSession.AuthenticationCode. Zaptec leaves that empty for
  remote-authorized sessions, which therefore read Unknown.
• Authorized — binary sensor, deliberately not diagnostic: with delayed
  charging the charger stays in Connected_Requesting after authorizing, so
  this is the only way to tell "authorized, waiting for the car" from "waiting
  for authorization" (Time delyaed charging and release status #420).

Where the installation does not require authorization the binary sensor reports
Unknown rather than "Not authorized", which would otherwise look like a fault
while the car charges normally.

The prefix rule lives in authorization.py, so both platforms derive their
value from it and cannot disagree.

Depends on #425: Zaptec clears 722 by sending it without a value, and without
that fix the token never clears.

Verified on a Go 2 (fw 3.3.0.0): 722 is not pushed over the live stream — 710
and 721 are — so it follows the poll interval. Authorizing from HA's own button
is picked up within a couple of seconds, since the button triggers a poll.

Fixes #200, #420

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