[feat][csharp][unity] Add unity 2019 support and update samples #847

Job	Run time
Build .Net projects (samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/)	2m 17s
Build .Net projects (samples/client/petstore/csharp/httpclient/standard2.0/Petstore/)	2m 15s
Build .Net projects (samples/client/petstore/csharp/generichost/standard2.0/Petstore/)	2m 18s
Build .Net projects (samples/client/petstore/csharp/restsharp/standard2.0/Petstore/)	2m 27s
	9m 17s

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