feat(Tools): add discord incoming webhook for sending messages

[yaoice]

Checklist:

Important

Please review the checklist below before submitting your pull request.

• Please open an issue before creating a PR or link to an existing issue
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

Description

Fixes #7851

Discord is great for playing games and chilling with friends, or even building a worldwide community. Add new tool that can send messages to Discord by using incoming webhook.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update, included: Dify Document
• Improvement, including but not limited to code refactoring, performance optimization, and UI/UX improvement
• Dependency upgrade

Testing Instructions

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration. Self check like following:

1. Create discord channel incoming webhook, and copy the webhook url

2. Add discord tool in dify workflow

3. Run dify workflow to send messages to discord channel

[yaoice]

@crazywoola help to approve th workflow to execute.