Generated code should use RequestContent.Create #5379
Description
The generator should emit code that uses RequestContent.Create, added in Azure/azure-sdk-for-net#51021.
Using ModelReaderWriter in Azure.Search.Documents, @m-nash noticed that the generated code isn't using RequestContent.Create, for example in AliasesRestClient.CreateCreateRequest.
Context: Azure/azure-sdk-for-net#51558 (comment)
Looks related to Azure/azure-sdk-for-net#51027.