Recipe input now accepts environment variables

In addition to config and value, recipe's input source element now accepts env. If source is env, there must also be an env element, which is the name of an environment variable to return.

The value will always be returned as a string. If the environment variable is not set, an empty string will be returned.

Example:

name: 'Input from environment variables'
input:
  name:
    data_type: string
    description: The name of the site.
    default:
      # Set the source to look for an environment variable.
      source: env
      # Define the name of the environment variable.
      env: SITE_NAME
  slogan:
    data_type: string
    description: The site slogan.
    default:
      source: env
      env: SITE_SLOGAN
config:
  actions:
    system.site:
      # Use the input in the same way we always have.
      simpleConfigUpdate:
        name: ${name}
        slogan: ${slogan}