Migrate Test-Json to System.Text.Json API

[iSazonov]

PR Summary

Before the change we used Newtonsoft Json.NET JObject.Parse() method to validate Json. The method is not best choice to validate Json because it doesn't parse valid primitives.

As result we:

• migrate the cmdlet to new Core API - System.Text.Json. JsonDocument.Parse()
• change old tests to follow JSON standard (use double quotes as delimiters instead of single quotes)
• add new tests
• enable nullable reference types to follow best practice

Formally it is a breaking change because Newtonsoft Json.NET JObject.Parse() accepted single quote delimiters.

PR Context

Fix #11384

Need review #5797

.Net docs https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-migrate-from-newtonsoft-how-to

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[iSazonov]

@mklement0 @jochenvanwylick @thedavecarroll @vexx32 The PR was moved to .Net Runtime 5.0 Preview1. Please play with compiled artifacts and feedback.

[vexx32]

Shouldn't these values be unquoted? Doesn't JSON accept true/false/null as primitive values?

[iSazonov]

Perhaps you thing about ConvertTo-Json. Test-Json accepts string input.

[vexx32]

Sure, but that's already tested in the case below. Is this not intended to verify that it accepts the true/false/null literals?

[iSazonov]

Ah, I see your point.
Add new tests

[ghost]

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[iSazonov]

Rebase to resolve merge conflicts.

[ghost]

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[iSazonov]

Rebased to move 5.0 RC2.

[ghost]

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[hilari0n]

Doesn't this change actually cause a performance drop as it causes the validated JSON to be parsed twice in ProcessRecord?
I.e. it's parsed first time explicitly, using JsonDocument.Parse(Json) (System.Text.Json parser), and then when _jschema.Validate(Json) is called (from NJsonSchema.JsonSchema, which internally parses it - I'm not sure, if it uses System.Text.Json parser or the Newtonsoft.Json parser).