Extract Envelope, DKIM, SPF, Charsets, Spam Score, and Spam Report in SG Adapter

These fields are useful for the following reasons:
 - Envelope: Only way to get the recipient email address in the case of a BCC
 - DKIM, SPF: Useful sender verification metrics from SendGrid
 - spam_score, spam_report: Useful spam metrics from SendGrid
 - charsets: for handling character encoding

Thanks for reviewing!