Add support for attachment meta steps

[baev]

Context

Due to limitations of the Allure 2 Model, attachments are always rendered at the end of the execution item. For example, the following code

await allure.logStep("step 1");
await allure.attachment("some data", "content", "text/plain");
await allure.logStep("step 2");

will show in the report:

1. step 1
2. step 2
3. some data

To workaround the issue, users may wrap attachments in steps, e.g. by adding @Step annotation on top of @Attachment method. However, such an approach will create redundant steps in the report. This pull request is fixing that, removing an extra step from the report.

Checklist

• Sign Allure CLA
• Provide unit tests