add realistic synthetic event generation for local testing

[sentry-junior]

add a cli workflow to generate fake but realistic Sentry events for local testing.

context from the thread:

• users want to test the local Sentry experience across different plans (for example AM1 and AM3) and see how more realistic events appear in-product
• current options are fragmented: sentry-cli send-event works for simple cases, SDK/demo apps can generate traffic, some folks replay stored event JSON, and getsentry/seer-test-sandbox has realistic scenarios but is aimed at Seer testing

proposal:

• add an explicit sentry-cli feature or subcommand for generating realistic synthetic events locally
• support richer payloads than a single simple error message
• ideally make it easy to vary event shape / complexity so product and engineering can exercise different UX paths without building custom apps each time

prior art mentioned:

• SENTRY_DSN="..." sentry-cli send-event --message "Test error from sentry-cli" --level error
• small SDK-based demo/test apps
• replaying stored event JSONs
• https://github.com/getsentry/seer-test-sandbox

raised by: Jason Swartz