[AIRFLOW-5113] Support icon url in slack web hook

[blcksrx]

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Description

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  the slack client has a feature to set the icon for webhook user and in airflow, it does not implement. so i added the icon_url on slack webhook and also on the slack operator

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Code Quality

• Passes flake8

[blcksrx]

@potiuk Hi! According to this PR, can you add the @AloPeyk comapny on the list of comapnies that uses airflow?

[potiuk]

Just open a PR adding it (it can be with AIRFLOW-XXX - no need to create JIRA