add synthetic event generation for local testing

- `sentry-cli send-event` is useful for simple event submission, but there is no obvious built-in way to generate fake-but-realistic event traffic for local Sentry testing. - this came up while testing the local product experience across plan tiers like AM1 and AM3, where realistic variation in event shape and attributes matters more than a single hand-crafted error. - prior art mentioned in discussion: using SDK-based demo apps, replaying stored event JSON from a small test app, and `getsentry/seer-test-sandbox` for realistic scenarios, but those are all heavier and repo-specific than a purpose-built CLI workflow. - adding a higher-level command or mode for synthetic/realistic event generation would make local testing and demos easier without requiring a custom app. Action taken on behalf of Jason Swartz.