Add support for class and collection xunit fixtures

Currently, there is no way to include a class- or collection-scoped fixture to a report produced by allure-xunit. They neither appear automatically nor could be included by `[AllureBefore]` or `[AllureAfter]` (applying the attributes causes the `Value cannot be null` error).

### Expected behavior

The following code:

```csharp
using Xunit;
using System;

public class ClassFixture : IDisposable
{
public ClassFixture(){}

public void Dispose(){}
}

public class CollectionFixture : IDisposable
{
public CollectionFixture(){}

public void Dispose(){}
}

[CollectionDefinition("My xunit collection")]
public class MyCollection : ICollectionFixture{}

[Collection("My xunit collection")]
public class TestClass : IClassFixture
{
public TestClass(ClassFixture _1, CollectionFixture _2){}

[Fact]
public void TestMehtod(){}
}
```

should produce the following report:


### Motivation

High-scope fixtures are important test framework constructs and ideally should be supported by allure out-of-box for all frameworks.

### Other considerations

1. There should be a way to exclude a fixture from the report
2. There should be a way to assign a custom name to a fixture (on a class or method levels)

See also #353.

Start by tracing how allure-xunit currently discovers and reports fixtures, using the supplied class- and collection-fixture example as the behavioral reference. Review the related discussion in #353; done means those fixtures appear in the report without the null error, with the requested exclusion and custom-naming behavior clarified.

Written by the indexing model from the issue text.