Add attchment support to SendGridAdapter

Summary:

• Add attachment support for SendGrid.
• Add attachment support to the MetaMTA test console.

Test Plan:

• Sent myself a file with Amazon SES via test console.
• Sent myself a file with SendGrid via test console.

Reviewers: mareksapota, jungejason, nh, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 1089