feat: Added support of replyToList in the library

[DivyamGoel]

Fixes

The replyToList support was added in v3 API but it has not been added in the library. In this PR, support for reply_to_list header has been added.

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added the necessary documentation about the functionality in the appropriate .md file
• [x] I have added inline documentation to the code I modified

If you have questions, please file a support ticket.

[ajeetdsouza]

Is there an ETA on this? We're using SendGrid at @CRED-CLUB and this an important feature for us.