Add script to sync Jenkins plugin versions from plugins.rb

[Crola1702]

Adds tools/update_plugin_versions.py, which reads a plugins.rb-style
attributes file (as used in
https://github.com/osrf/chef-osrf/blob/latest/cookbooks/ros2ci/attributes/plugins.rb)
and updates matching plugin="name@version" references across the empy
templates under job_templates/, recursively

Usage:

python3 tools/update_plugin_versions.py <path-to-plugins.rb> [templates_dir] [--dry-run]

The plugins.rb versions can be regenerated from the live Jenkins server with
https://github.com/osrf/chef-osrf/blob/latest/scripts/jenkins_plugins.rb:

./scripts/jenkins_plugins.rb <Server> <User> <Password/Token>

This should make future plugin-version syncs (like #897) a single command
instead of manual per-file edits.