def_preferred.json - Introduce a preferred def.json file for use on newer clients

Specification:

• New agents with this functionality will read def_preferred.json from the same folder as def.json, if it exists.
• Old clients will ignore this file.
• Both def.json and def_preferred.json have the same syntax and support the same features, but you can use newer functionality in the preferred file, which is ignored by older clients.
• A new command line option --ignore-preferred-augments will disable this functionality
  • In the future, when all your clients support this feature, it allows you to distinguish which clients use the normal vs preferred JSON file.
  • Can be added to exec command, systemd service, etc.

Rationale:

This is being introduced because we are adding new functionality to the augments format as part of 3.18.0 and want some ways for users who still have old 3.15 clients to use the new functionality on their 3.18+ hosts.