Description
We would really like to add some more personalizations (https://sendgrid.com/docs/for-developers/sending-email/personalizations/) to our Sendgrid emails, but we're not sure what would the best approach for a PR. The only way I can think of to pass that extra data to the SendgridApiTransport class is by a custom json-encoded header on the Email like "x-sendgrid-personalizations" that we would then merge with the personalizations array created in the SendgridApiTransport class?