[TASK][EASY] Support setting the username to launch Kyuubi engines in USER & CONNECTION share level

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Skill requirements

• Basic knowledge on Scala Programing Language

Background and Goals

Hive supports SQL operation authorization against the session user, and then runs SQL as Hive Server user.

Currently, Kyuubi supports this feature at the SERVER share level.

We want to also support this feature in other share levels, which provide fine isolation.

Implementation steps

Currently, EngineRef#appUser serves two purposes:

• The username used to run the SparkSQLEngine
• Part of the key used to look up the SparkSQLEngine to connect to.

To implement this feature, we need to replace EngineRef#appUser with two new fields, EngineRef#appUser and EngineRef#engineSpaceUser.

• The new EngineRef#appUser is the username used to run the SparkSQLEngine.
• The EngineRef#engineSpaceUser is used to look up the SparkSQLEngine.

EngineRef#engineSpaceUser is set in the same way as the old EngineRef#appUser.

The new EngineRef#appUser is set according to a new config, kyuubi.engine.user:

• When kyuubi.engine.user is not set, set EngineRef#appUser to EngineRef#engineSpaceUser
• When kyuubi.engine.user is set, set EngineRef#appUser to that value.

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