Allows serialization of null valued properties.

[timayabi2020]

Fixes #2609 and #3008

Changes proposed in this pull request

• Adds autorest directive to ensure that null valued properties are not omitted by default.
• Adds a custom JsonExtensions class containing an extension method that ensures properties explicitly defined as null in a PowerShell session are retained
• Adds test project to test the above custom class.

This PR has a dependency on the autorest update done here which is part of addressing the issue here microsoftgraph/autorest.powershell#9