Add SshCommand.ExecuteAsync (#1426)

* Add SshCommand.ExecuteAsync

After the previous change (#1423), this basically entails swapping out the
IAsyncResult for a TaskCompletionSource and hooking up the cancellation/timeout
logic.

As with the prior Begin/End implementation, the initiation of the command
(SendExecRequest) happens synchronously, so there's a bit of room for improvement
there, but otherwise it is the Task-based async that we know and like.

I chose to make it void (Task)- returning instead of string like in the existing
overloads, so that OutputStream is not automatically consumed (and encoded as a
string) when that may not be desired. As in #650, I was initially considering
changing the other overloads to be void-returning as well, but decided that it was
not worth the break since most people will probably want to change over to
ExecuteAsync anyway.

* Update examples

---------

Co-authored-by: Wojciech Nagórski <wojtpl2@gmail.com>

docfx/examples.md

@@ -113,16 +113,17 @@ using (var client = new SshClient("sftp.foo.com", "guest", "pwd"))
     // Make the server echo back the input file with "cat"
     using (SshCommand command = client.CreateCommand("cat"))
     {
-        IAsyncResult asyncResult = command.BeginExecute();
+        Task executeTask = command.ExecuteAsync(CancellationToken.None);
 
         using (Stream inputStream = command.CreateInputStream())
         {
             inputStream.Write("Hello World!"u8);
         }
 
-        string result = command.EndExecute(asyncResult);
+        await executeTask;
 
-        Console.WriteLine(result); // "Hello World!"
+        Console.WriteLine(command.ExitStatus); // 0
+        Console.WriteLine(command.Result); // "Hello World!"
     }
 }
 ```

src/Renci.SshNet/.editorconfig

@@ -180,3 +180,11 @@ dotnet_diagnostic.CA5350.severity = none
 # CA5351: Do Not Use Broken Cryptographic Algorithms
 # https://learn.microsoft.com/en-ca/dotnet/fundamentals/code-analysis/quality-rules/ca5351
 dotnet_diagnostic.CA5351.severity = none
+
+# MA0040: Forward the CancellationToken parameter to methods that take one
+# Partial/noisy duplicate of CA2016
+dotnet_diagnostic.MA0040.severity = none
+
+# MA0042: Do not use blocking calls in an async method
+# duplicate of CA1849
+dotnet_diagnostic.MA0042.severity = none