[Feature] generate subclient with public constructor

[chunyu3]

Current .NET SDK support two kinds of Client:

• service Client
• sub client : has no public constructor, it only can be created by the parent.

There is a third kind of client: a subclient with public constructor

Rule: When a subclient with public constructor:

• the subclient name will suffix with Client.
• The subclient will has additional public client constructor

example as following:

• FaceServiceClient is the service client
• FaceListClient is the subclient which has public client constructor

pubic partial class FaceServiceClient
{
    public FaceServiceClient(Uri endpoint, AzureKeyCredential credential, FaceServiceClientOptions options)
    {
            Argument.AssertNotNull(endpoint, nameof(endpoint));
            Argument.AssertNotNull(credential, nameof(credential));
            options ??= new AnomalyDetectorClientOptions();

            ClientDiagnostics = new ClientDiagnostics(options, true);
            _keyCredential = credential;
            _pipeline = HttpPipelineBuilder.Build(options, Array.Empty<HttpPipelinePolicy>(), new HttpPipelinePolicy[] { new AzureKeyCredentialPolicy(_keyCredential, AuthorizationHeader) }, new ResponseClassifier());
            _endpoint = endpoint;
            _apiVersion = options.Version;
    }
    FaceListClient GetFaceListClient()
    FaceLargeListClient GetLargeFaceListClient()
    PersonGroupClient GetPersonGroupClient()
    PersonLargeGroupClient GetLargePersonGroupClient()
}

public partial class FaceListClient
{
  //added public constructor
  public FaceListClient(Uri endpoint, AzureKeyCredential credential, FaceServiceClientOptions options)
  {
            Argument.AssertNotNull(endpoint, nameof(endpoint));
            Argument.AssertNotNull(credential, nameof(credential));
            options ??= new AnomalyDetectorClientOptions();

            ClientDiagnostics = new ClientDiagnostics(options, true);
            _keyCredential = credential;
            _pipeline = HttpPipelineBuilder.Build(options, Array.Empty<HttpPipelinePolicy>(), new HttpPipelinePolicy[] { new AzureKeyCredentialPolicy(_keyCredential, AuthorizationHeader) }, new ResponseClassifier());
            _endpoint = endpoint;
            _apiVersion = options.Version;
  }
  
  // interal construtor for Get accessor
  internal FaceServiceClient(ClientDiagnostics clientDiagnostics, HttpPipeline pipeline, AzureKeyCredential keyCredential, Uri endpoint, string apiVersion)
  {
      ClientDiagnostics = clientDiagnostics;
      _pipeline = pipeline;
      _keyCredential = keyCredential;
      _endpoint = endpoint;
      _apiVersion = apiVersion;
  }
  
}

[chunyu3]

explore augmenting the @clientInitializer decorator onto a namespace or interface which could convert it to a SubClient with a public initializer
Codegen will generate this client as above:

• client name suffix with Client
• add an extra public client constructor which can initialize itself

Then there will be three kind of client:

• service Client

• sub client : has no public constructor, it only can be created by the parent.

• sub client with public constructor: it has public constructor, it can be created by parent, also can be created by itself.

[JoshLove-msft]

This is done