Attachments for the confirmation emails

[SeanCassiere]

Support the following email attachment-related features.

1. Including the mandatory global documents assigned to the confirmation email template.
   i. The response list for this will be put in the key docListForAttach of the request body in the email submission.
   ii. The documents to be included can be obtained from /api/v3/Emails/AttachmentsForComposeEmail with the following query params.

{
  "ClientId": 1013,
  "TemplateTypeId": 1, // this will change based on the metadata retrieved for the confirmation email template
  "TemplateId": 123411
}

2. Including the uploading documents as attachments for the confirmation email template.
   i. The submission of these attachments should be configurable based on the store/controller type. (License Images, ...etc.)
   ii. These attachments will be put in the key attachments of the request body in the email submission, for which the list shape will be as follows.

{
  "foo": "bar",
  "attachments": [{ "fileName": "string", "blob": "base64 string -> data:image/png;base64,iV...." }]
}