Added SumAsync

[wertzui]

The SumAsync method was missing from the Entity Framework Core extension methods.

[StefH]

Unit tests fail.

Because the SumAsync has multiple overloads. See https://docs.microsoft.com/en-us/dotnet/api/system.data.entity.queryableextensions.sumasync?view=entity-framework-6.2.0

[StefH]

please take a look at my comments

[StefH]

Also add a new test file EntitiesTests.Sum.cs to test this new code.

[codecov]

Codecov Report

Merging #290 into master will decrease coverage by 0.24%.
The diff coverage is 71.73%.

@@            Coverage Diff             @@
##           master     #290      +/-   ##
==========================================
- Coverage    87.5%   87.25%   -0.25%     
==========================================
  Files          42       42              
  Lines        4280     4324      +44     
==========================================
+ Hits         3745     3773      +28     
- Misses        535      551      +16

Impacted Files	Coverage Δ
...rkCore.DynamicLinq/EFDynamicQueryableExtensions.cs	72.86% <71.73%> (-0.04%)	⬇️
...rc/System.Linq.Dynamic.Core/DynamicClassFactory.cs	96.81% <0%> (-1.82%)	⬇️

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[StefH]

@wertzui Can you maybe add a SumAsync exmple to https://github.com/StefH/System.Linq.Dynamic.Core/blob/master/src-console/ConsoleAppEF1.1/Program.cs ? In that way you can be sure that this works for NETCore 1.x