feat(logging.xunit): add provider alias to xunit logger provider

[stijnmoreels]

With multiple logger providers, it is sometimes the case that you want to filter certain logs based on category with specific log levels. To do this for custom logger providers, you can set the [ProviderAlias(...)] attribute.

This PR sets this attribute to the custom logger provider of the xUnit logging package.
This would allow stuff like:

{
  "Logging": {
    "LogLevel": {
      "Default": "Information"
    },
    "Xunit": {
      "LogLevel": {
        "Default": "Trace"
      }
    }
  }
}

Relates to #242

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🧪 Code coverage summary

Metric	Value
Line coverage	🟢 91.9%
Branch coverage	🟢 81.3%

Great job! 😎 Your code coverage is higher than my caffeine levels! ☕