chore(github): add CODEOWNERS for PR review assignments

[Sunny6889]

Summary

This PR introduces a .github/CODEOWNERS file to enforce structured, module-level code review across the java-tron repository.

• Reviewers are selected based on community activity — contributors with a sustained record of meaningful commits, reviews, and issue engagement in their respective areas
• Each top-level Gradle module (framework, chainbase, actuator, consensus, protocol, common, crypto, plugins) has dedicated owners
• Critical sub-packages (db, net, vm/tvm, jsonrpc, rpc, http, event, config, backup, metrics, rewards, lite) are further refined with domain experts
• Test code mirrors the ownership of the corresponding production module
• A default fallback owner is defined for any path not covered by a specific rule
• Rules follow GitHub's last-match-wins precedence: more specific paths are placed further down the file

Motivation

As the codebase grows, ad-hoc reviewer selection leads to inconsistent review quality. By anchoring ownership to demonstrated community involvement, we ensure PRs are reviewed by contributors who are actively familiar with the affected code, improving review reliability and reducing the risk of subtle regressions.