Add preliminary pubsub and bucket trigger support

[ssttevee]

This PR is not necessarily ready to be merged, I'm more looking for some comments and discussion.

To handle a pubsub or bucket trigger, use nodego.UseEventHandler() instead of http.Handle():

nodego.UseEventHandler(func(event *nodego.FunctionEvent) error {
    msg, err := event.PubSubMessage()
    if err != nil {
        return err
    }

    // OR

    evt, err := event.StorageEvent()
    if err != nil {
        return err
    }

    return nil
})

[ssttevee]

I'm uncertain about StorageEvent fields, whether or not it should be included since I probably haven't included all the possible json object properties.

[iangudger]

I think this is the official parsing struct for StorageEvent: https://github.com/google/google-api-go-client/blob/master/storage/v1/storage-gen.go#L1224

[ssttevee]

Do you think I should import it and use it directly even though it includes other unused fields? Likewise with the PubSubMessage: https://github.com/google/google-api-go-client/blob/master/pubsub/v1/pubsub-gen.go#L730

[iangudger]

That would probably be best.

[iangudger]

Any progress?

[ssttevee]

It seems to have been working fine on my own functions for now. Probably okay to be merged.

[iangudger]

Do you want to do the cleanup we discussed?

[ssttevee]

Do you not see the changes? I'm pretty sure they have been pushed.

[ssttevee]

There's one issue I've noticed with this PR that I'm still pondering.

The testing feature on the web ui (as well as gcloud beta functions call) does not work when using a non-http hook because that calls the /execute route where as the /execute/ route is being used. This works in js because express takes the /execute* route which does not work with the std http mux.

[iangudger]

With regard to the route, maybe we should just change the user code listener to accept /.

I think it would be better to put the new examples in the examples directory instead of examples/triggers.

[googlebot]

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[googlebot]

CLAs look good, thanks!

[ssttevee]

Sorry, messed up some git commands...

I added support for the new event context: https://cloud.google.com/functions/docs/writing/background

[ssttevee]

I'm not sure if the nodego import from the events package should be an absolute or relative import. Depends if the intended use case is to import this package or to use this package as a starting point.