Introduce yiisoft/db-implementation virtual package

[vjik]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	✔️
Fix #698

Related PRs:

• Provide yiisoft/db-implementation db-mssql#372
• Provide yiisoft/db-implementation db-mysql#405
• Provide yiisoft/db-implementation db-oracle#336
• Provide yiisoft/db-implementation db-pgsql#420
• Provide yiisoft/db-implementation db-sqlite#368
• Provide yiisoft/db-implementation dummy-provider#13

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.00%. Comparing base (f550106) to head (e064be8).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #998      +/-   ##
============================================
+ Coverage     92.31%   99.00%   +6.69%     
  Complexity     1677     1677              
============================================
  Files           102      102              
  Lines          4240     4240              
============================================
+ Hits           3914     4198     +284     
+ Misses          326       42     -284     

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