[NETCore31] Add support for using System.Text.Json as the serialization parser

[normj]

Issue #, if available:
#544

Description of changes:
.NET Core 3.0 added a new JSON serialization called System.Text.Json. This PR creates new Lambda serialization called Amazon.Lambda.Serialization.SystemTextJson that will be based on System.Text.Json. Unlike Newtonsoft's JSON parser System.Text.Json is built into the framework and is optimized for speed.

This will released as part of the upcoming Lambda .NET Core 3.1 and will only target .NET Core 3.1. There will not be a .NET Standard 2.0 which would make it available to .NET Core 2.1 Lambda functions because the event packages like Amazon.Lambda.S3 would have to take a dependency on the System.Text.Json Nuget package.

In my anecdotal testing I have seen some good cold start performance improvements using this library. For those that are currently trying .NET Core 3.1 on Lambda using the custom runtime feature here are preview versions of the impacted NuGet packages to try out.

https://normj-packages.s3.us-west-2.amazonaws.com/System.Text.Json-2020-01-17.zip

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

[msimpsonnz]

There is a .NET Standard package for this which makes it available to 2.1
Would be nice if the SDK’s could align
https://www.nuget.org/packages/System.Text.Json

[martincostello]

Are there more annotations that could be added that would make it possible to remove this? It comes with a performance penalty to some degree, so would be better if it could be left off.

[normj]

I was thinking users could use the constructor that takes in Action<JsonSerializerOptions> customizer to customize or remove these settings. I'm open to suggestions to make it easier.

[normj]

@msimpsonnz That was my original attention to use the NuGet package for .NET Core 2.1. The problem is the AWS event packages require customization of the .NET Property name to JSON Property name mapping. For Newtonsoft we could add System.Runtime.Serialization.DataMember attributes to the event packages like Amazon.Lambda.CloudWatchLogsEvents to handle the customization. For System.Text.Json it doesn't recognize System.Runtime.Serialization.DataMember attributes and instead you have to use System.Text.Json.Serialization.JsonPropertyName. That means in order to support System.Text.Json for .NET Core 2.1 Lambda functions the event packages would have to take a NuGet package dependency on a System.Text.Json. I don't want to force anybody using .NET Core 2.1 to have to include any other NuGet package especially if they are not using it because they are using the Newtonsoft serializer.