feature: PubSub triggers

[orthros]

Similarly to Cron Support; there are events from other services (e.g. Kokoro) whose job completion may trigger the invocation of a bot.

Proposed changes:

Bots

• A bot that wants to subscribe to PubSub triggers needs to include a topic file in their source folder
• A bot that wishes to subscribe to PubSub triggers needs to subscribe to the topic.repository event in the appFn:

 app.on('topic.repository', async context => {});

Infrastructure

• The build process will inspect all bots && if a bot has a topic file and no corresponding PubSub topic it will
  • Create a PubSub topic whose name is the bot name
  • Create a Push Subscription for the topic and point it at the Cloud Run serverless-scheduler-proxy instance. NOTE: This will use a custom ServiceAccount that has permissions to invoke the CloudRun instance.

serverless-scheduler-proxy

• Move the cron processing endpoint to /cron (was /)
• Add a new endpoint /pubsub which handles incoming PubSub push messages of form:

// PubSubMessage is the payload of a Pub/Sub event.
type PubSubMessage struct {
        Message struct {
                Data []byte `json:"data,omitempty"`
                ID   string `json:"id"`
        } `json:"message"`
        Subscription string `json:"subscription"`
}

• This handler parses the name of the bot from the Subscription field, and creates a new HTTP request to the bot/Cloud Function whose name is in the subscription. This request will be signed using the signing key of the bot in the x-hub-signature header, and the x-github-event header will be topic.repository