add constructor that allows changing the serializer settings

[mabead]

Issue #500

Description of changes:

As described in related issue, I add a constructor that makes it easy to customize the lambda json serialization settings.

Here's an example of how the json serialization settings can be changed by using a derived class:

public class MySerializer : JsonSerializer
{
    public MySerializer()
        : base(CustomizeSerializerSettings)
    {
    }

    private static void CustomizeSerializerSettings(JsonSerializerSettings serializerSettings)
    {
        serializerSettings.Converters = new List<JsonConverter>
        {
            new StringEnumConverter() { CamelCaseText = false }
        };
    }
}

The new serializer could then be used like this:

[assembly: LambdaSerializer(typeof(MySerializer))]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[normj]

Version 1.6.0 of Amazon.Lambda.Serialization.Json is out with the PR. Thanks for the PR!

[UrmaGurd]

Is there an example of how to use this to deserialize strings to enums with Lambda functions?

My lambda function is currently using

[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]

How do I change this to support enums / string conversions?