feat: add support for reading message payload from bucket

Part of #36 (closed)

Description

This MR adds support reading the message payload from a bucket. It accepts INPUT_BUCKET and INPUT_FILE as environment variables, reads the content and passes that to the handlers, in a similar way how it would have been done in the normal ucam-faas workflow.

Example run can be seen here (message was {"foo":"bar"}):

https://cloudlogging.app.goo.gl/9JMmYbv8XiUsdfxK8

How to test?

Since I deployed the example image to ucam-faas-long development terraform, you can test it yourself:

1. Go to the Google Cloud Console
2. Go to the faas-test pub/sub topic
3. Send in a message
4. Go to the fl-test-pubsub Cloud Run Job and confirm in the logs that it got the event