feat(triggers): support custom webhook body as requested in #230

[rajnandan1]

This pull request includes several changes to enhance the functionality and documentation of the webhook triggers. The most important changes include updating the documentation to include variable placeholders, adding support for custom webhook bodies, and updating the version number.

Documentation updates:

• docs/triggers.md: Added variable placeholders to the webhook body description and included examples for customizing the webhook body and sending messages to a Telegram channel. [1] [2]

Codebase enhancements:

• src/lib/components/manage/triggerInfo.svelte: Added fields for custom webhook body and validation for webhook headers. Added a placeholder for the custom webhook body. [1] [2] [3] [4] [5] [6]
• src/lib/server/notification/notif.js: Updated the Notification class to handle the new webhook metadata structure.
• src/lib/server/notification/webhook.js: Modified the Webhook class to support custom webhook bodies and replace secrets in the body. [1] [2] [3]

Version update:

• package.json and src/routes/(docs)/+layout.svelte: Updated the version number from 3.0.6 to 3.0.7. [1] [2]