Add CompositeExpression

[vjik]

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	❌

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.40%. Comparing base (13e8b4d) to head (5ff68cc).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1057   +/-   ##
=========================================
  Coverage     98.40%   98.40%           
- Complexity     1634     1639    +5     
=========================================
  Files           115      117    +2     
  Lines          4323     4333   +10     
=========================================
+ Hits           4254     4264   +10     
  Misses           69       69           

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[Tigrov]

Could you also add doc here https://github.com/yiisoft/db/blob/master/docs/guide/en/expressions/expressions.md
to better understand how to use it