Add a Cloud Fuctions callback

[KeisukeYamashita]

What

• Add Cloud Functions as a backend handler
• Supporting Cloud Pub/Sub is much better
• Similar to Add a lambda callback, for using lambda functions in handlers #8

[ryan-lane]

Hey, would be great to have this support! I don't actively develop against gcp currently, but would love to have this support added and would be more than willing to add the abstractions to make it easier