Allow to insert values via a select query

[Baptouuuu]

Request

formal-php/orm#45 requires the ability to insert rows for multiple aggregates at once. In SQL this translates to the insert into select queries with virtual columns containing the values to insert.

Implementation

• Select->columns() now accepts Row\Value to express virtual columns with a specified value.
• Insert::into() now accepts either a Row\Value or a Select query

Both features combined allows the ORM to build queries to insert values that come from both the storage and provided user values.

[codecov]

Codecov Report

Attention: Patch coverage is 97.87234% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.57%. Comparing base (b795786) to head (0a09a18).
Report is 10 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/Query/Insert.php	92.85%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop      #17      +/-   ##
=============================================
- Coverage      97.59%   97.57%   -0.02%     
- Complexity       375      385      +10     
=============================================
  Files             38       38              
  Lines           2160     2190      +30     
=============================================
+ Hits            2108     2137      +29     
- Misses            52       53       +1     

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