Make variable entity_id available to value_template for MQTT binary sensor

[emontnemery]

Description:

#18501, which proposed adding "toggle" functionality to binary sensor, was closed because the same thing should already be possible to achieve with a value template.

Doing it with a value template however requires hard coding the entity_id, which is tedious and error prone in case of manual edit of configuration.yaml, and impossible if the entity is automatically discovered.

This PR makes variable entity_id available to the value_template, in the same manner as it is already available to automation templates.

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#8021

Example entry for configuration.yaml (if applicable):

# Toggle the binary sensor each time a message is received on state_topic
binary_sensor:
  - platform: mqtt
    state_topic:lab_button/cmnd/POWER"
    value_template:"{%if is_state(entity_id,\"on\")-%}OFF{%-else-%}ON{%-endif%}"}

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

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

• Documentation added/updated in home-assistant.io

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[balloob]

Ok to merge when test added.