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 98.32%. Comparing base (2ecc3da) to head (07ccc10). Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #353      +/-   ##
============================================
+ Coverage     98.27%   98.32%   +0.04%     
- Complexity      222      227       +5     
============================================
  Files            19       19              
  Lines           695      715      +20     
============================================
+ Hits            683      703      +20     
  Misses           12       12              

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