Add a "Send email via Mailtrap" CDP destination

Feature request

Mailtrap already has a data warehouse source in the Posthog catalog, thanks for that, and we'd love to add a bit more.

The main thing we'd like to build: a "Send email via Mailtrap" CDP destination, so PostHog events can trigger Mailtrap sends. It'd mirror your existing mailgun and sendgrid templates, so it's self-contained and easy to review. If you're up for it, two more we'd happily PR:

• Expand our existing source with the endpoints your COVERAGE_GAPS_APPENDIX flags as missing (campaigns and stats, /stats/date, contact members, per contact events). That adds engagement data, opens, clicks, bounces.
• Move the source from alpha to GA (and add OAuth) once that lands.

Would you be open to us opening these?

Thanks!

Debug info

- [ ] PostHog Cloud, region and project ID: [please provide, you can find both at https://app.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]

Compare the existing Mailgun and SendGrid CDP destination templates first; the issue says the Mailtrap destination should mirror them. Verify that the destination can trigger Mailtrap sends from PostHog events and remains self-contained and easy to review. The additional source coverage and GA/OAuth work are separate follow-ups.