NLogLoggingConfiguration - Added support for NLog variables

[snakefoot]

Example JSON-config:

{
    "NLog": {
        "variables": {
            "var_layout": "${message}",
            "var_encoding": "utf-8"
        },
        "targets": {
            "console": {
                "type": "Console",
                "layout": "${var_layout}"
            },
            "file": {
                "type": "File",
                "layout": "${var_layout}",
                "encoding": "${var_encoding}",
                "fileName": "c:/temp/console-debug.log"
            }
        },
        "rules": [
            {
                "logger": "*",
                "minLevel": "Trace",
                "writeTo": "file,Console"
            }
        ]
    }
}

[codecov]

Codecov Report

Merging #282 into master will increase coverage by 0.53%.
The diff coverage is 87.17%.

@@            Coverage Diff             @@
##           master     #282      +/-   ##
==========================================
+ Coverage   69.92%   70.45%   +0.53%     
==========================================
  Files          12       12              
  Lines         941      958      +17     
  Branches      165      170       +5     
==========================================
+ Hits          658      675      +17     
  Misses        221      221              
  Partials       62       62

Impacted Files	Coverage Δ
...ensions.Logging/Config/NLogLoggingConfiguration.cs	84.21% <87.17%> (+4.54%)	⬆️

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[snakefoot]

@304NotModified Ready for review. Would like to continue on with default-wrapper and default-target-parameters