Jenkins Slack Notification Plugin missing permission check

Jenkins Slack Notification Plugin did not perform permission checks on a method implementing form validation. This allowed users with Overall/Read access to Jenkins to connect to an attacker-specified URL using attacker-specified credentials IDs obtained through another method, capturing credentials stored in Jenkins.

Additionally, this form validation method did not require POST requests, resulting in a cross-site request forgery vulnerability.

This form validation method now requires POST requests and Overall/Administer (for global configuration) or Item/Configure permissions (for job configuration).