Remove dependency on System.Linq.Async

Added ToListAsync implementation to eliminate dependency on
System.Linq.Async, which provided System.Linq.AsyncEnumerable.ToListAsync.

System.Linq.Async introduces extension methods to a default (or even
mandatory) namespace which conflict with other implementations, see:
dotnet/reactive#1057

src/Novell.Directory.Ldap.NETStandard/Novell.Directory.Ldap.NETStandard.csproj

@@ -57,7 +57,6 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="System.Linq.Async" Version="5.0.0" />
     <PackageReference Include="AsyncFixer" Version="1.1.6">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
@@ -74,4 +73,4 @@
     -->
   </ItemGroup>
 
-</Project>
+</Project>

src/Novell.Directory.Ldap.NETStandard/Utilclass/AsyncEnumerableExtensions.cs

@@ -0,0 +1,21 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+public static class AsyncEnumerableExtensions
+{
+    /// <summary>
+    /// Asynchronously materializes the subject <see cref="IAsyncEnumerable{T}"/> into a list.
+    /// </summary>
+    public static async Task<List<T>> ToListAsync<T>(this IAsyncEnumerable<T> enumerable, CancellationToken cancellationToken = default)
+    {
+        var list = new List<T>();
+        await foreach (var element in enumerable.WithCancellation(cancellationToken))
+        {
+            list.Add(element);
+        }
+
+        return list;
+    }
+}