Add webhook for Lettermint

[thasmo]

This PR adds a webhook integration for Lettermint, a transactional email provider. A mailer adapter for Craft CMS is already available in the plugin store.

• webhook endpoint at /actions/campaign/webhook/lettermint that handles Lettermint events
• Supports hard bounces and spam complaints - marks contacts accordingly
• Webhook signature validation using HMAC SHA-256 (follows their signing docs)
• New lettermintWebhookSigningSecret setting for the signing secret (supports environment variables)
• Webhook URL now shows up in the settings page
• Includes tests for both signed and unsigned requests

The webhook listens for these events:

• webhook.test
• message.hard_bounced
• message.spam_complaint

Signature validation is optional (controlled by the existing validateWebhookRequests setting) and uses the same security approach as the other webhook integrations.

[bencroker]

Thanks for the PR. I’ll need at least a description before I can take this seriously.

[thasmo]

Thanks for the PR. I’ll need at least a description before I can take this seriously.

Absolutely, I will add a description ASAP.

[thasmo]

@bencroker, I've added a PR description.