Changes:

Adds a new RPKI validation stage with support for RTR protocol and file-based ROA loading
Refactors variable naming from opt_* prefix to simpler names across multiple stages
Consolidates utility functions from multiple locations into a new pkg/util package
Enhances error handling in file operations with proper error logging
Updates dependency versions and adds new dependencies for RPKI functionality

This PR refactors network connection handling and configuration management across the BGP pipeline stages, with improvements to the RPKI/RTR client implementation and build tooling.

Changes:

Refactored connection handling to centralize configuration options in util.DialRetry and renamed ConnHandle to ConnBGP
Improved RTR client session management with better locking patterns and state initialization
Enhanced shell script robustness in the release build process

This PR implements a new "split" mode for RPKI validation that separates invalid prefixes into a dedicated withdrawal message, along with refactoring to improve code consistency and upgrading dependencies.