JSON condition and evaluator

[Sebbs128]

Investigate creating a JSON-specific evaluator for request bodies.

Using System.Text.Json, this might look like either using the PipeReader (per ASP.NET Core docs advice on request operations, or passing the body stream to a Utf8JsonReader after enabling request buffering.

Care needs to be taken with the stream approach to ensure YARP can still correctly forward the request contents.

There is some advice at https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/use-utf8jsonreader, which also includes a section on using Utf8JsonReader with a ReadOnlySequence<byte>, which is applicable to the PipeReader option of implementing this.

Configuration should likely contain a way to specify a path for the particular json property to be evaluated against MatchValues