Make CustomerUpdatePackagePubKey optional in PeblarSystemInformation.

Some white-label Peblar devices (for example ChargePoint-branded variants) do not include this field in /api/v1/system/information. The current strict model crashes deserialization with a mashumaro.exceptions.MissingField error.

• Change customer_update_package_public_key from required str to optional str | None with default=None.
• Add regression test that:
  • loads a realistic system_information payload fixture,
  • removes CustomerUpdatePackagePubKey,
  • verifies parsing still succeeds and value becomes None.
• Add fixture file used by that regression test.

This keeps backward compatibility for devices that return the field, while making parsing tolerant for white-label firmware variants that omit it.

My white-label (Chargepoint) charger has been connected to HA succesfully and has been stable for the past days