Feat/support sendgrid

[andrematteo]

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Summary

Included support for sending emails via the Twilio SendGrid platform.
Created the file libs/SendGridClient
Modified extensions/Mail
Modified config/Feature/init
Modified .env.example

close #21010

Tested using a free SendGrid account.

Screenshots

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Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[crazywoola]

Please fix the errors in the CI pipeline.

[crazywoola]

• Please run ./dev/reformat to resolve lint errors.
• And fix the tests accordingly, link: https://github.com/langgenius/dify/actions/runs/15683302504/job/44239535942?pr=21011

[crazywoola]

Merged :)