Support environment variables DSC Meta Configuration

[Gijsreyn]

Summary of the new feature / enhancement

With the initial batch completed in #574, I want to be able to use environment variables in the directories field. The current implementation in preview.12 doesn't support the following:

{
  "resourcePath": {
    "allowEnvOverride": true,
    "appendEnvPath": true,
    "directories": ["%SYSTEMDRIVE%\\CompanyResources"]
  },
  "tracing": {
    "level": "TRACE",
    "format": "Default",
    "allowOverride": true
  }
}

Proposed technical implementation details (optional)

No response

[SteveL-MSFT]

The problem is that env vars have different syntax depending on the shell and depending on the OS. Maybe we can solve this by allowing expressions in the settings file which has an env() function that could be used along with path().

[Gijsreyn]

Ah that's right. Would love to see it as an expression!

[michaeltlombardi]

Personally, I lean away from complicating the configuration files to support expressions. In my opinion, the configuration file should be explicit and readable purely as a text representation of the data, especially for integration/inspection scenarios.

I also wonder about the security/safety implications of allowing the meta configuration to be overridden by editing an environment variable.

[SteveL-MSFT]

Env vars would probably be necessary in cases particularly on Windows that accesses the filesystem. You can't assume the systemdrive is c: and technically %programfiles%, %programdata%, %userprofile%, etc... could be moved to non-standard locations. I would agree we would not recommend using custom env vars as a way to modify config behavior, that should be parameters.