Realize `Schema::loadResultColumn()` method

[Tigrov]

Related PR

• https://github.com/yiisoft/db/pull/945

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	✔️/❌
Fixed issues

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.49%. Comparing base (e170b66) to head (1ecd71a). Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #348      +/-   ##
============================================
+ Coverage     98.47%   98.49%   +0.01%     
- Complexity      330      336       +6     
============================================
  Files            21       21              
  Lines          1052     1063      +11     
============================================
+ Hits           1036     1047      +11     
  Misses           16       16              

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