Support send_each_at mail header

[yudppp]

Issue Summary

I want to use send_each_at mail header.But send_each_at header is not support now.
send_each_at is scheduling-parameters
Reference
https://docs.sendgrid.com/for-developers/sending-email/scheduling-parameters

Steps to Reproduce

1. This is the first step
2. This is the second step
3. Further steps, etc.

Code Snippet

# paste code here

Exception/Log

# paste exception/log here

Technical details:

• sendgrid-go version:
• go version:

[yudppp]

I implemented #447

[JenniferMah]

Hi @yudppp! Thanks for submitting a PR! Our team has added it to our backlog for review.

[ervitis]

Hi @JenniferMah, it has passed a little time since @yudppp opened the issue and the pull request with a solution. Is the project on maintenance or is still active?
Thanks a lot for the hard work.