Add tuta-sdk as submodule, use SDK blob reading API

- Add tuta-repo as git submodule pointing to spartanz51/tutanota
  branch feat/rust-sdk-blob-read (pending upstream PR)
- Remove inline load_mail_details_blob hack, call through
  mail_facade().load_mail_details_blob() instead
- Remove /tuta-repo from .gitignore since it's now a submodule

src/tuta.rs

@@ -85,7 +85,7 @@ impl TutaSession {
         mail: &Mail,
     ) -> Result<Option<MailDetailsBlob>, ApiCallError> {
         if mail.mailDetails.is_some() {
-            let blob = self.logged_in.load_mail_details_blob(mail).await?;
+            let blob = self.logged_in.mail_facade().load_mail_details_blob(mail).await?;
             Ok(Some(blob))
         } else {
             Ok(None)