add model name mapping option to rust generators #871

Job	Run time
Build .Net projects (samples/client/petstore/csharp/generichost/standard2.0/Petstore/)	3m 39s
Build .Net projects (samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/)	3m 33s
Build .Net projects (samples/client/petstore/csharp/httpclient/standard2.0/Petstore/)	3m 38s
Build .Net projects (samples/client/petstore/csharp/restsharp/standard2.0/Petstore/)	3m 25s
	14m 15s

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