Given:

1. cluster configuration with have extra flag <impersonate_users>1</impersonate_users>
2. default user has a special impersonate permission

Scenario:

1. user filimonov connect to server A and run a query on Distributed table.
2. server A connects to server B & server C with its own credentials (let's say as user 'default')
3. the query send from server A to other servers have an extra setting: SELECT ... SETTINGS user='filimonov' (because of <impersonate_users>1</impersonate_users>)
4. Target server dynamically change the default user to filimonov (because it allowed by impersonate permission).
5. all user restrictions for filimonov user are applied (row-based security, allow databases, etc).

Later extra scenarios like "execute particular matview as user XXX" can be added.

Inspired by

• https://docs.microsoft.com/en-us/dotnet/framework/data/adonet/sql/customizing-permissions-with-impersonation-in-sql-server
• https://docs.microsoft.com/en-us/sql/t-sql/statements/execute-as-transact-sql?view=sql-server-ver15
• sudo :)

Allows to solve the following issues:

1. using interserver connection pool w/o reconnects
2. one-time user authentication (no need to store / pass / rerequest authentication information to pass forward)

Related: #6843 #8926