Add SshCommand.ExecuteAsync

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

As with the prior Begin/End pattern, 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 sshnet#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.

Author: Rob-Hague

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