[C#] Add ChannelCredentials.SecureSsl property for better codecompletion with ChannelCredentials (#26472)

* Add ChannelCredentials.Secure

* Update

* Test

* PR feedback

src/csharp/Grpc.Core.Api/ChannelCredentials.cs

@@ -31,6 +31,7 @@ namespace Grpc.Core
     public abstract class ChannelCredentials
     {
         static readonly ChannelCredentials InsecureInstance = new InsecureCredentials();
+        static readonly ChannelCredentials SecureSslInstance = new SslCredentials();
 
         /// <summary>
         /// Creates a new instance of channel credentials
@@ -51,6 +52,22 @@ namespace Grpc.Core
             }
         }
 
+        /// <summary>
+        /// Returns instance of credentials that provides SSL security.
+        /// <para>
+        /// These credentials are the same as creating <see cref="SslCredentials"/> without parameters.
+        /// Apps that are using Grpc.Core can create <see cref="SslCredentials"/> directly to customize
+        /// the secure SSL credentials.
+        /// </para>
+        /// </summary>
+        public static ChannelCredentials SecureSsl
+        {
+            get
+            {
+                return SecureSslInstance;
+            }
+        }
+
         /// <summary>
         /// Creates a new instance of <c>ChannelCredentials</c> class by composing
         /// given channel credentials with call credentials.

src/csharp/Grpc.Core.Tests/ChannelCredentialsTest.cs

@@ -30,6 +30,12 @@ namespace Grpc.Core.Tests
             Assert.IsFalse(ChannelCredentials.Insecure.IsComposable);
         }
 
+        [Test]
+        public void SecureCredentials_IsComposable()
+        {
+            Assert.IsTrue(ChannelCredentials.SecureSsl.IsComposable);
+        }
+
         [Test]
         public void ChannelCredentials_CreateComposite()
         {
@@ -52,6 +58,10 @@ namespace Grpc.Core.Tests
             var nativeCreds1 = creds.ToNativeCredentials();
             var nativeCreds2 = creds.ToNativeCredentials();
             Assert.AreSame(nativeCreds1, nativeCreds2);
+
+            var nativeCreds3 = ChannelCredentials.SecureSsl.ToNativeCredentials();
+            var nativeCreds4 = ChannelCredentials.SecureSsl.ToNativeCredentials();
+            Assert.AreSame(nativeCreds3, nativeCreds4);
         }
     }
 }