fix(*): gRPC and cJSON's behavior to empty array

[StarlightIbuki]

Summary

Lua does not distinguish empty arrays and tables, which makes empty array decoded from gRPC gateway be encoded as {} by cJSON.
The fix makes use of cjson.empty_array_mt to hint the cJSON.
Special thanks to @starwing, who spent their public holidays adapting new features for this issue.

Checklist

• The Pull Request has tests
• There's an entry in the CHANGELOG
• N/A There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Full changelog

• Bumped lua-protobuf
• Use cjson.empty_array_t for arrays
• Tests

Issue reference

starwing/lua-protobuf#240
Fix FTI-5002
Fix #10801

Review suggestion

1. Please go through the discussion add metatable to decoded array and message to tell array from map starwing/lua-protobuf#240 before reviewing;
2. *.pb.go are auto-generated

[chronolaw]

Need a person familiar with Go to double-check.

[fffonion]

two non-technical minor issues, otherwise lgtm

[flrgh]

rebased onto master to fix conflicts; good to merge whenever CI is 🟢

[kikito]

This fix was added very late so it was missing a the "cherrypick into 3.3" label (notice that this PR targets master). I just added it