Add `AsyncEnumerable.GroupBy` extensions #535

Job	Run time
csproj-build (Release, true)	43s
csproj-build (Release, false)	45s
csproj-build (Release_NoProgress, true)	42s
csproj-build (Release_NoProgress, false)	49s
csproj-build (Debug, true)	44s
csproj-build (Debug, false)	46s
csproj-build (Debug_NoProgress, true)	43s
csproj-build (Debug_NoProgress, false)	47s
	5m 59s

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