feat(cli) exposing underlying serf keys commands

[subnetmarco]

Summary

Exposes the underlying serf keys in Kong, to:

perform cluster-wide encryption key operations, such as installing new keys and removing old keys. When used properly, the keys command allows you to achieve non-disruptive encryption key rotation across a Serf cluster.

Full changelog

• Adds a new cluster_keyring_file property in the configuration file.
• Exposes serf keys .. commands in the kong cluster keys .. CLI command to perform cluster-wide encryption key operations:
  • kong cluster keys list
  • kong cluster keys install <key>
  • kong cluster keys remove <key>
  • kong cluster keys use <key>