Feature Request: Ability to specify http.Agent in configuration

[sluramod]

Issue Summary

We are using sendgrid api in firebase environment. firebase cloud functions have various quotas, including quotas for outbound connections. The document is here: https://firebase.google.com/docs/functions/networking#maintaining_persistent_connections
Right now there is no way to specify httpAgent to use in sendgrid configuration.

Technical details:

Ideally, it would be great to do something like this:

const httpAgent = new https.Agent({ keepAlive: true });

sendgrid.setHttpAgent(httpAgent)
...
sendgrid.send({args})

[JenniferMah]

Pull requests to add this feature are welcome and will be reviewed based on priority, but Twilio SendGrid is not actively building new functionality for the library.