Refactor arguments to command resources an mixed strings and objects for extensibility

[SteveL-MSFT]

PR Summary

• args are now always processed regardless if how input is specified, so it's technically possible to have the input passed as both an arg and stdin/env
• args now take a string or an object where the object currently supported looks like:

{
  "jsonInputArg": "<parameter>",
  "mandatory": true
}

The value of jsonInputArg is what the input gets passed to, so "jsonInputArg": "--foo" would be passed as command --foo $json effectively.

The mandatory indicates whether to pass the JSON even if empty. Default is false so it skips providing this parameter completely, but if true, it will always provide the parameter and an empty string if input is empty.

PR Context

Fix https://github.com/PowerShell/DSC/issues/218