v0.21.0-beta.2: New connection handling

Additional fix:

• Access Token check (fixes: #292)

IMPORTANT:

• This is a test release and it contains a new logic for the connection handling. Please be aware of potential bugs and provide feedback.
• It can take 24 hours until the new logic is effective. During this time, you might experience account blocks, particularly if you update the component in the afternoon. (See here for some test results)
• Please check the homeassistant.log and/or enable the debug logging before you file an issue. (The help page is using MQTT as an example, please replace MQTT with MBAPI2020)

Main goal: Prevent the account blocking with a new connection handling

New connection handling:

• the connection is closed after 30 seconds if no update messages are received from the MB server. If a message is received or a car's ignition state is on, the 30 seconds window restarts.
• the connection is re-established after 60 seconds.
• changes that have happened in the meantime are delivered by the MB systems. (the latest values, not all)
• if an action, button or switch is triggered, the connection remains active for 120 seconds in order to receive long-running responses from the MB system.

Other fixes:

• New capability modes for auxHeat (fixes: #288, some cars reporting that the command capabilities for auxheat are not available, but at the same time the car reports the capability auxHeat. We check "auxHeat" also now.)