feat: generate cloudevent types from googleapis/google-cloudevents (#371)

This commit adds a new `npm run generate_cloudevents` script that
generates cloudevent interfaces from the schemas in
googleapis/google-cloudevents

Author: matthewrobertson

experimental/generate_cloudevents/README.md

@@ -0,0 +1,11 @@
+# Code Generation Pipeline For CloudEvent Interfaces
+
+This directory contains an experimental code generation pipeline to generate TS interfaces for the CloudEvent types defined in [Google Events](https://github.com/googleapis/google-cloudevents).
+
+It can be run via the following command:
+
+```bash
+npm run generate_cloudevents
+```
+
+This will regenerate all known CloudEvent type interfaces in the `src/cloudevent_types` directory of this repository.