Add a webhook automation trigger

[balloob]

Description:

This adds a webhook automation trigger. Supports both JSON and POST requests. Allows users to register webhooks and have that data be routed to their automations.

Allows users to easily send data in their local network to Home Assistant in a secure way without dealing with all the auth stuff.

Example entry for configuration.yaml (if applicable):

automation:
  trigger:
    platform: webhook
    webhook_id: hello-super-webhook
  action:
    service: light.turn_on
    data_template:
      entity_id: '{{ trigger.json.entity_id }}'

And then from your external scripts send JSON or POST data to http://hassio.local:8123/api/webhook/hello-super-webhook

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If the code does not interact with devices:

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