Add default constructor for CustomField

[sfkoch]

I've been trying to configure Serilog.Sinks.Splunk.EventCollectorSink from appsettings.json, but can't get it to construct the CustomFields object. Among other things, I tried structuring "fields" for ConfigurationBinding.Get. That failed because CustomField doesn't have a default constructor.

Please add a default constructor on CustomField so that it can be deserialized from JSON like this:

        "fields": {
          "CustomFieldList": [
            {
              "Name": "name1",
              "ValueList": [ "value1" ]
            },
            {
              "Name": "name2",
              "ValueList": [ "value2" ]
            },
            {
              "Name": "name3",
              "ValueList": [ "value3" ]
            }
          ]
        }