feat: add msglib receive adapter

internal/isphere/source.go

@@ -22,6 +22,16 @@ type Message struct {
 	Subject          string
 	ReceiptID        string
 	Read             bool
+	Source           string
+	RawRef           string
+	Attachments      []MessageAttachment
+}
+
+type MessageAttachment struct {
+	FileID      string
+	FileName    string
+	SizeBytes   *int64
+	DownloadRef string
 }
 
 type ReceiveMessagesQuery struct {