Add @allure.manual decorator to simplify ALLURE_MANUAL=True label setting

[betapl3b]

Context

[//]: # ( Describe the problem or feature in addition to a link to the issues ) For now, to mark your test case as a manual, you need to use decorator @allure.label("ALLURE_MANUAL", True). This thing is not intuitive and not even stated in documentation.

This PR is adding @allure.manual decorator which is replacing this usage.

Checklist

• [x] Sign Allure CLA
• [x] Provide unit tests

[CLAassistant]

All committers have signed the CLA.

[skhomuti]

@betapl3b thanks!