Split PostgreSQL specific code into separate project

[VMelnalksnis]

Split off PostgreSQL specific code into Gnomeshade.Data.PostgreSQL project, so that is is easier to add support for other dabases, for example, SQLite.

[codecov]

Codecov Report

Merging #426 (5620627) into master (fbb3b5d) will increase coverage by 0.14%.
The diff coverage is 88.42%.

@@            Coverage Diff             @@
##           master     #426      +/-   ##
==========================================
+ Coverage   64.39%   64.54%   +0.14%     
==========================================
  Files         256      258       +2     
  Lines        6831     6836       +5     
  Branches      521      532      +11     
==========================================
+ Hits         4399     4412      +13     
+ Misses       2274     2262      -12     
- Partials      158      162       +4     

Impacted Files	Coverage Δ
...igrations/20220313170407_InitialCreate.Designer.cs	100.00% <ø> (ø)
...grations/PostgreSQLIdentityContextModelSnapshot.cs	0.00% <ø> (ø)
source/Gnomeshade.Data/Identity/ApplicationUser.cs	100.00% <ø> (ø)
...nomeshade.Data/Migrations/DatabaseUpgradeLogger.cs	66.66% <ø> (ø)
...e.Data/Repositories/AccountInCurrencyRepository.cs	75.00% <ø> (ø)
.../Gnomeshade.Data/Repositories/AccountRepository.cs	100.00% <ø> (ø)
...Gnomeshade.Data/Repositories/CategoryRepository.cs	100.00% <ø> (ø)
...eshade.Data/Repositories/CounterpartyRepository.cs	90.90% <ø> (ø)
...rce/Gnomeshade.Data/Repositories/LinkRepository.cs	100.00% <ø> (ø)
...rce/Gnomeshade.Data/Repositories/LoanRepository.cs	92.85% <ø> (ø)
... and 45 more

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[VMelnalksnis]

Had to fiddle with migrations, both EF and dbup, in order to retain existing migration names and not cause issues with existing server installations.