Use concrete RestResponse in generator

[RikkiGibson]

Closes #153. Will need to merge Azure/autorest-clientruntime-for-java#393 and publish a snapshot before CI will pass.

[RikkiGibson]

> linux-x64 restart

[RikkiGibson]

> win32-x64 restart

[olydis]

🤖 AutoRest automatic feature coverage report 🤖

feature set version 2.3.14

❌️ General: 92%

38 out of 522 features are not covered by tests

ConstantsInBody, ConstantsInPath, FileStreamVeryLarge, FormdataStreamUploadFile, HttpRedirect300Get, HttpRedirect300Head, HttpRedirect301Get, HttpRedirect301Put, HttpRedirect302Get, HttpRedirect302Head, HttpRedirect302Patch, HttpRedirect303Post, HttpRedirect307Delete, HttpRedirect307Get, HttpRedirect307Head, HttpRedirect307Patch, HttpRedirect307Post, HttpRedirect307Put, ResponsesScenarioC200MatchingModel, ResponsesScenarioC201MatchingModel, ResponsesScenarioC404MatchingModel, StreamUploadFile, UrlPathsArrayCSVInPath, UrlQueriesArrayMultiEmpty, UrlQueriesArrayMultiNull, UrlQueriesArrayMultiValid, allowedValueEnum, expectedEnum, getByteEmpty, getByteInvalid, getComplexPolymorphismComplicated, getEnumReferenced, getEnumReferencedConstant, putComplexPolymorphismComplicated, putEnumReferenced, putEnumReferencedConstant, roundTripEnum, unexpectedEnum

❌️ Azure: 94%

7 out of 118 features are not covered by tests

AzureMethodQueryUrlEncoding, AzureODataFilter, AzurePathQueryUrlEncoding, AzureRequestClientIdInError, AzureSwaggerQueryUrlEncoding, AzureXmsRequestClientIdNull, LRONonRetryPut201Creating400InvalidJson