feat(external users): email sanity + mock SMTP

- SMTP has a mock to enable end to end test and validate the whole path
     (via OXICLOUD_SMTP_MOCK)
    - add email normalisation ( including punicode)
    - api to share to external user

tests/api/run.sh

@@ -101,7 +101,8 @@ hurl --variables-file "$API_DIR/test.env" --file-root "$REPO_ROOT/tests" --test
   "$API_DIR/permissions.hurl" \
   "$API_DIR/grants.hurl" \
   "$API_DIR/subject_groups.hurl" \
-  "$API_DIR/grants_nested_groups.hurl"
+  "$API_DIR/grants_nested_groups.hurl" \
+  "$API_DIR/external_users.hurl"
 
 #bash "$API_DIR/dedup_bulk_upload.sh"