Add `environment` field to code function handler

[schickling]

The code function handler should accept a environment string:string-map like in the following example:

functions:

  auth:
    type: schemaExtension
    schema: my-schema.graphql
    handler:
      code:
        src: "my-script.js"
        environment:
          AUTH0_MASTER_TOKEN: prefix-${env:MY_AUTH0_TOKEN}