onnxruntime

Add ability to register custom ops from ORT extensions nuget package (#15696)

### Description
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Add infrastructure so it's easy for a user to add the ORT extensions
nuget package and register the custom ops for C# apps.

### Motivation and Context
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- If it fixes an open issue, please link to the issue here. -->
Need to be able to use extensions on mobile platforms with Xamarin/MAUI