Add attachment support to JUnit XML plugin

[chrisnikoli]

Context

Allows adding attachments from system-out in the format [[ATTACHMENT|path/to/file]]
Here are some examples of others using this format:
https://plugins.jenkins.io/junit-attachments/
https://devblogs.microsoft.com/devops/junit-attachments-support-for-publish-test-results/
https://docs.gitlab.com/ee/ci/testing/unit_test_reports.html#view-junit-screenshots-on-gitlab

Checklist

• Sign Allure CLA
• Provide unit tests

[delatrie]

Hello, @chrisnikoli
Thank you for your contribution. This feature will be implemented in Allure 3. Here is the issue to track: allure-framework/allure3#80. We may backport it to Allure 2 later though.