Update serialization to use System.Text.Json (#32)

[jamdavil]

Description

This PR updates the API Client to use the modern System.Text.Json rather than System.Web.Script.Serialization as an initial step to modernize the library with an upgrade to net standard and / or the latest runtime, resolving #32 .

Motivation and Context

Resolve issue #32 and remove a blocker to targeting a more modern .NET runtime.

How Has This Been Tested?

In addition to ensuring unit tests still pass, I also ran the example program against a test Admin API. I inspected each of the data types returned in the resulting dictionary to verify that they have not changed between master and this branch, which is the biggest risk of this change as the payloads of various API endpoints aren't defined with formal types, just the generic dictionary.

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)