[http-client-csharp] emitter support for @clientDefaultValue on model properties

The Azure C# emitter does not currently apply client-side default values for model properties marked with the legacy `@clientDefaultValue` decorator. The Spector test `Azure_ClientGenerator_Core_ClientDefaultValue_PutModelProperty` (in `azure-http-specs`) constructs `new ModelWithDefaultValues("test")` and expects `timeout=30`, `tier="standard"`, `retry=true` to be auto-populated and serialized, but only `name` is sent.`r`n`r`nThe test is currently `[Ignore]`-d in `ClientDefaultValueTests.cs` pending emitter support.