Make allure-xmlunit

You have https://github.com/allure-framework/allure-java/tree/master/allure-jsonunit , but we really need same feature for xml, please

Start by reviewing the existing allure-jsonunit directory in the repository to understand the referenced feature and how it is organized. Define the XML counterpart's scope from that implementation, with equivalent XML support as the completion criterion; the issue does not name specific files or tests.

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