[Cognitive Services] Makes Id/Name/Type fields of CognitiveServicesAccount readonly

[felixwa]

Makes Id/Name/Type fields readonly by making set private.

Description

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[felixwa]

Note: I did not update AssemblyInfo.cs nor *.csproj because we have not released 2.0.0 version and this change is in scope of 2.0.0.

[felixwa]

Consequences of Azure/azure-rest-api-specs#1589