Add current_user alias

[yariks5s]

Added current_user as a compatibility alias for MySQL.

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• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Added current_user function as a compatibility alias for MySQL.

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

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[alexey-milovidov]

Add a test.

[johnnymatthews]

Don't forget to add docs to https://clickhouse.com/docs/en/sql-reference/functions/other-functions#currentUser.

[johnnymatthews]

Approving the docs section of this PR. I'll leave it to the engineers to approve the actual code.

[yariks5s]

The current test turned out to be flaky. As the previous check of alias here was simple, I will make similar checks instead and will investigate the root cause of this strange behavior in one of the next PRs.