Add performance tests for JsonNode and related types

[ahsonkhan]

We should add performance tests to https://github.com/dotnet/performance and compare scenarios against Newtonsoft.Json.

This includes scenarios such as :

• JsonNode.Parse (versus JsonDocument.Parse)
• JsonNode.DeepCopy
• Create JsonElement, modify some properties, and then convert back to JsonElement.
• Enumerate all the nodes/tokens from a JsonNode (versus JsonDocument/JsonElement).

cc @kasiabulat, @joperezr

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