Get rid of allure-xunit's [AllureXunit] and [AllureXunitTheory] attributes

[delatrie]

Currently, in order to use allure-xunit one have to:

1. Install the package
2. Create allureConfig.json
3. Replace all [Fact] and [Theory] attributes with [AllureXunit] and [AllureXunitTheory].

The goal of this issue is to get rid of the 3rd step.

Motivation

The end goal is to make allure-xunit usable out-of-box. A user should be able to generate the report by just installing the package (optionally, adding some configuration) and running the tests.

Negative implications of using [AllureXunit] and [AllureXunitTheory]

The need to replace attributes has two major negative implications:

1. It's a time-consuming operation if a user has a fairly large number of tests.
2. The [AllureXunit] and [AllureXunitTheory] attributes cannot be composed with other attributes derived from [Fact] or [Theory]. If a user decides to utilize such attributes from a 3rd party library, there will be no way to use allure-xunit at all.

Example

1. Current usage:

   using Allure.Xunit.Attributes;
   using Xunit;
   
   public class TestClass
   {
       [AllureXunit]
       public void TestMethod() { }
   
       [AllureXunitTheory]
       [InlineData(1, "a")]
       [InlineData(2, "b")]
       public void ParameterizedTestMethod(int a, string b) { }
   }

2. Future usage:

   using Xunit;
   
   public class TestClass
   {
       [Fact]
       public void TestMethod(){}
   
       [Theory]
       [InlineData(1, "a")]
       [InlineData(2, "b")]
       public void ParameterizedTestMethod(int a, string b) { }
   }

The same report should be created for both usages:

Backward compatibility

Allure-xunit must still recognize the [AllureXunit] and [AllureXunitTheory] attributes correctly and collect test results from the corresponding test methods.