Add event_trigger to cloud functions

[rileykarson]

Deprecate old trigger_ objects, and retry_on_failure.

Decisions:

• Let's over-required on failure_policy.retry to stop weird configs
• resource expects mostly a partial self link, except gcs buckets use a different shape of self link. The shape Cloud Functions expect is not that shape. The shape they use is made up, so there is an error if you supply a gcs self link... let's be restrictive and only allow names for now. We need to specify the shape if we take in names, so we need to match against specific event_types. So let's only support the two trigger types we supported before for now (Cloud Storage, Cloud Pub/Sub)
• Note for the future: Formats are documented here
• event_type accepts a specific pattern that is documented. It will also accept some different formats (at least a certain subset of the pattern) and is not as restrictive as it seems, but let's not document that because the Cloud Functions documentation does not. And also we need users to match specific shapes so we can format the self links correctly.

[ghost]

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