feat: add mail_v3 functionality for reply_to_list

[lopezator]

Add reply_to_list functionality. It just implements a feature that is available on api but not yet on the SDK (and should).

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• 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
• I have added inline documentation to the code I modified

[lopezator]

Tested empirically sending email using my service and worked fine.

[sarinarw]

bump - would like to have this feature added

[sarinarw]

I also tested this by importing it into my project and it successfully sent an email with multiple reply-tos

replace github.com/sendgrid/sendgrid-go => github.com/lopezator/sendgrid-go v0.0.0-20231219091217-c76dfc3e8156

Need someone with actual access to review - but LGTM FWIW