Add runtime per-bucket pub/sub notification configuration API

[drehelis]

Following #2131,

I was thinking that maybe runtime configuration is a better approach to add support for multiple pub/sub notification configurations. Extending proposed solution also to per-bucket notification.

How this works:

1. Start fake-gcs server

PUBSUB_EMULATOR_HOST=host.docker.internal:8085 go run main.go -scheme http -port 4443 --filesystem-root ./storage

2. Create a bucket

curl -s -X POST http://localhost:4443/storage/v1/b \
  -H 'Content-Type: application/json' \
  -d '{"name": "my-bucket"}'
  
3. Add a notification config for OBJECT_FINALIZE events

curl -s -X POST http://localhost:4443/storage/v1/b/my-bucket/notificationConfigs \
  -H 'Content-Type: application/json' \
  -d '{
    "topic": "projects/my-project/topics/my-topic",
    "payload_format": "JSON_API_V1",
    "event_types": ["OBJECT_FINALIZE"],
    "object_name_prefix": "uploads/"
  }'

4. Trigger an event

curl -s -X POST \
  "http://localhost:4443/upload/storage/v1/b/my-bucket/o?uploadType=media&name=uploads/test.txt" \
  -H 'Content-Type: text/plain' \
  -d 'hello world'

Result:

Supported events are: OBJECT_FINALIZE, OBJECT_DELETE, OBJECT_METADATA_UPDATE, OBJECT_ARCHIVE

Full disclosure:
I'm the author of gcp-emulator-ui which is also a fake-gcs UI interface and a runtime configuration is better suited then the proposed init config in #2135

[drehelis]

Hey @fsouza, can you have another look?

[fsouza]

Thank you!