Add unity 2019 support and update samples (#21036) #848

Job	Run time
Build .Net projects (samples/client/petstore/csharp/generichost/standard2.0/Petstore/)	2m 54s
Build .Net projects (samples/client/petstore/csharp/httpclient/standard2.0/Petstore/)	1m 32s
Build .Net projects (samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/)	2m 59s
Build .Net projects (samples/client/petstore/csharp/restsharp/standard2.0/Petstore/)	2m 50s
	10m 15s

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