Support decimal precision for Document.FromJson and ToJson by using System.Text.Json instead of LitJson

[ashovlin]

Description

Before

Currently, Document.FromJson and Document.ToJson in DynamoDBv2 use LitJson for JSON handling.

This does not support decimal, as the first numeric type it matches to here is double.

aws-sdk-net/sdk/src/Core/ThirdParty/Json/JsonReader.cs

Lines 132 to 144 in c88243b

	if (number.IndexOf ('.') != -1 ||
	number.IndexOf ('e') != -1 ||
	number.IndexOf ('E') != -1) {
	double n_double;
	if (Double.TryParse(number, NumberStyles.Any, CultureInfo.InvariantCulture, out n_double))
	{
	token = JsonToken.Double;
	token_value = n_double;
	return;
	}
	}

This is leading to a loss of precision for decimal values, see the linked issues.

After

This proposes to address this by replacing LitJson in these code paths with System.TextJson.

• We want to do this anyway as long term we'd like to remove the embedded copy of LitJson from AWSSDK.Core.

While this addresses the reported issue, we still need to look at:

• Is this causing a regression for any other numeric values or types, or even non-numeric types?
• Is it handling comments, malformed JSON, etc. the same way?

Motivation and Context

#1555 , #1039

Testing

• Ran DynamoDBv2 sln tests locally
• PENDING dry-run

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license

[ashovlin]

Not sure why this is included, didn't port it over for now.

[ashovlin]

This test was passing before despite the missing comma, but STJ throws an exception now.