Hide self from step parameters by default #222 - deliver mentioned parameter

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#### 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?
self and cls parameters are deleted from step(allure.step) in report

#### If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

#### What is the expected behavior?
I would like have self or cls in "step" print (allure.step). I would like to have special parameter mentioned in fixed issue https://github.com/allure-framework/allure-python/issues/222 or any other method to achieve this

#### What is the motivation / use case for changing the behavior?
"self" is important in my framework because it have __repr__ and i would like to see it in step parameters in report

#### Please tell us about your environment:

- Allure version: 2.14.0
- Test framework: pytest@6.2.4
- Allure adaptor: allure-pytest@2.9.43

#### Other information

[//]: # (
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Start at the allure.step entry point and inspect how self and cls parameters are removed from the report. Review the behavior requested in the linked allure-python issue #222, then identify the relevant tests or add coverage for the chosen approach. Done means self or cls can be included in step parameters and their representation is visible in the report.

索引モデルが issue の本文から書いたものです。