add support for custom_unsubscribe_url to sendgrid adapter

[mogmarsh]

Description

Sendgrid supports a custom_unsubscribe_url or suppression_group_id (but not both at the same time). We have UI for suppression_group_id. It would be nice to have UI for the custom_unsubscribe_url and then logic to choose one over the other. Or maybe it should be a filter so that only one ever shows up and the implementor can choose which one to use?

Use Case

Users should be able to specify a custom_unsubscribe_url and have it used instead of suppression_group_id