Add .proto ext to check-license-header.sh

Add `//` as the comment style in `.proto` files.

Author: rnro

.github/workflows/scripts/check-license-header.sh

@@ -67,6 +67,7 @@ while IFS= read -r file_path; do
     json) continue ;; # JSON doesn't support comments
     jsx) expected_file_header=$(sed -e 's|@@|//|g' <<<"${expected_file_header_template}") ;;
     kts) expected_file_header=$(sed -e 's|@@|//|g' <<<"${expected_file_header_template}") ;;
+    proto) expected_file_header=$(sed -e 's|@@|//|g' <<<"${expected_file_header_template}") ;;
     ps1) expected_file_header=$(sed -e 's|@@|##|g' <<<"${expected_file_header_template}") ;;
     py) expected_file_header=$(cat <(echo '#!/usr/bin/env python3') <(sed -e 's|@@|##|g' <<<"${expected_file_header_template}")) ;;
     rb) expected_file_header=$(cat <(echo '#!/usr/bin/env ruby') <(sed -e 's|@@|##|g' <<<"${expected_file_header_template}")) ;;