Expose RestClient from ClientProvider (microsoft#5888)

So that, we can get the corresponding RestClientProvider while
generating Resource in Azure plugin.
Corresponding PR in Azure plugin:
Azure/azure-sdk-for-net#47944

packages/http-client-csharp/generator/Microsoft.Generator.CSharp.ClientModel/src/Providers/ClientProvider.cs

@@ -196,7 +196,10 @@ private IReadOnlyList<ParameterProvider> GetClientParameters()
             return null;
         }
 
-        internal RestClientProvider RestClient => _restClient ??= new RestClientProvider(_inputClient, this);
+        /// <summary>
+        /// Gets the corresponding <see cref="RestClientProvider"/> for this client.
+        /// </summary>
+        public RestClientProvider RestClient => _restClient ??= new RestClientProvider(_inputClient, this);
         internal Lazy<ClientOptionsProvider?> ClientOptions { get; }
 
         public PropertyProvider PipelineProperty { get; }