Discuss and add support for public webhook based trigger

Description

Discuss with team and design how ucam-faas can accept webhooks as a trigger type.

This will involve:

• Updating the python library to allow for webhooks to be used
• Updating the terraform module to allow for this trigger type to be specified

The discussion/design for this issue may result in this being a non-starter/non-option. In this case, discuss with team and close issue.

Further details

Task list

Acceptance criteria

• Discuss and review how this will work with the currently existing decorators
• new trigger type available
  • can be public or private (must be public, private support optional)
• basic alerting added to similar level
• python library/docker image supports http trigger and returning a value and status codes
• Deployed webhook trigger to example/dev environment in ucam-faas terraform module