Add support for "reply_to_list"

[illing2005]

Please add support for the new feature that one can specify a multiply reply to addresses in the mail header.

This is supported via the reply_to_list attribute of the mail/send endpoint:
https://docs.sendgrid.com/api-reference/mail-send/mail-send#multiple-reply-to-emails

[eshanholtz]

This issue has been added to our internal backlog to be prioritized. Pull requests and +1s on the issue summary will help it move up the backlog.

[nbeuchat]

That would be really important for us as well!