xUnit.net: Support for [Trait]

#### I'm submitting a ...
- [ ] bug report
- [x] feature request
- [ ] support request => Please do not submit support request here, see note at the top of this template.

#### What is the current behavior?
The `[Trait]` attribute isn't shown in the Allure reporter in any way.

#### What is the expected behavior?
Traits are automatically converted according to some algorithm.

#### What is the motivation / use case for changing the behavior?
The `[Trait]` attribute is commonly used to organize tests into groups. IDEs use traits to provide better navigation through tests.

#### Please tell us about your environment:

- Allure version: 2.1.0
- Test framework: testng@6.8
- Allure adaptor: allure-testng@2.0-BETA8
- Generate report using: allure-maven@2.18

#### Other information

The following options should be considered first:

1. Convert all traits to tags.
2. Convert specific traits (e.g., `Category`) to tags.
3. Convert all traits to labels.
4. Convert specific traits to specific labels.
5. Convert specific traits to links.
6. Configure traits to labels/links according to the config.

Start by resolving which of the six proposed conversions applies to xUnit.net [Trait] values, then inspect the Allure reporter entry point responsible for trait metadata. Done means the chosen mapping is implemented and verified in generated reports.

Written by the indexing model from the issue text.