ISSUE-2994: add function current_user()

[flaneur2020]

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Summary

this PR depends on #2986

this PR adds a current_user() function, to test the session's current user

Changelog

• Improvement

Related Issues

Fixes #2994

Test Plan

Unit Tests
Stateless Tests

[databend-bot]

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[codecov-commenter]

Codecov Report

Merging #2995 (0a58baa) into main (5eec227) will increase coverage by 0%.
The diff coverage is 35%.

@@          Coverage Diff          @@
##            main   #2995   +/-   ##
=====================================
  Coverage     68%     68%           
=====================================
  Files        650     651    +1     
  Lines      34293   34324   +31     
=====================================
+ Hits       23517   23542   +25     
- Misses     10776   10782    +6     

Impacted Files	Coverage Δ
common/planners/src/plan_expression.rs	83% <ø> (ø)
common/functions/src/scalars/udfs/current_user.rs	16% <16%> (ø)
common/functions/src/scalars/udfs/udf.rs	90% <100%> (+<1%)	⬆️
query/src/functions/context_function.rs	93% <100%> (+1%)	⬆️
query/src/functions/context_function_test.rs	92% <100%> (+2%)	⬆️
common/management/src/cluster/cluster_mgr.rs	78% <0%> (-3%)	⬇️
query/src/clusters/cluster.rs	54% <0%> (-1%)	⬇️
query/src/common/mod.rs	83% <0%> (+12%)	⬆️

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[BohuTANG]

/lgtm

[BohuTANG]

This need a document issue :D

[flaneur2020]

@BohuTANG thank you reminding, i've created a document issue for this: #3006 qwq