Add device class support to MQTT cover

[krconv]

What does this implement/fix?

Adds device_class support in discovery for MQTT covers. Similar to #1832, which did the same thing for MQTT sensors.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Related issue or feature (if applicable): fixes esphome/issues#2023 and fixes esphome/feature-requests#1158

Pull request in esphome-docs with documentation (if applicable): N/A

Test Environment

• ESP32
• ESP8266

Example entry for config.yaml:

mqtt:
  id: mqtt_client
  broker: !secret broker
  username: !secret mqtt_username
  password: !secret mqtt_password

switch:
  - platform: gpio
    pin: D3
    name: "Garage Door Open Switch"
    id: open_switch
  - platform: gpio
    pin: D4
    name: "Garage Door Close Switch"
    id: close_switch

cover:
  - platform: template
    device_class: garage
    name: "Garage Door"
    open_action:
      - switch.turn_off: close_switch
      - switch.turn_on: open_switch
      - delay: 0.1s
      - switch.turn_off: open_switch
    close_action:
      - switch.turn_off: open_switch
      - switch.turn_on: close_switch
      - delay: 0.1s
      - switch.turn_off: close_switch
    stop_action:
      - switch.turn_off: close_switch
      - switch.turn_off: open_switch
    optimistic: true
    assumed_state: true

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[krconv]

I checked through the rest of the MQTT components, and I think covers + sensors are the only ones with device_class

[OttoWinter]

Thanks 👍