terminal: add a simple TTY-based menu system

Add a simple menu system that works over the TTY. This is useful for
presenting a selection of authentication options to the user when no UI
is available.

Author: mjcheetham

src/shared/Microsoft.Git.CredentialManager/TerminalMenu.cs

@@ -0,0 +1,109 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace Microsoft.Git.CredentialManager
+{
+    public class TerminalMenu : IEnumerable<TerminalMenuItem>
+    {
+        private readonly ITerminal _terminal;
+        private readonly IList<TerminalMenuItem> _items = new List<TerminalMenuItem>();
+
+        public TerminalMenu(ITerminal terminal, string title = null)
+        {
+            _terminal = terminal;
+            Title = title;
+        }
+
+        public string Title { get; }
+
+        public void Add(TerminalMenuItem item)
+        {
+            if (item.IsDefault && _items.Any(x => x.IsDefault))
+            {
+                throw new ArgumentException("A default menu item has already been added.");
+            }
+
+            _items.Add(item);
+        }
+
+        public int Show()
+        {
+            TerminalMenuItem defaultItem = _items.FirstOrDefault(x => x.IsDefault);
+            bool hasDefault = !(defaultItem is null);
+
+            string title = $"{Title ?? "Select an option"}:";
+
+            string promptDefaultTag = hasDefault ? " (enter for default)" : string.Empty;
+            string prompt = $"option{promptDefaultTag}";
+
+            while (true)
+            {
+                _terminal.WriteLine(title);
+
+                foreach (TerminalMenuItem item in _items)
+                {
+                    string itemDefaultTag = item.IsDefault ? " (default)" : string.Empty;
+                    _terminal.WriteLine($"  {item.Id}. {item.Name}{itemDefaultTag}");
+                }
+
+                string optionStr = _terminal.Prompt(prompt);
+
+                if (string.IsNullOrWhiteSpace(optionStr))
+                {
+                    if (hasDefault)
+                    {
+                        return defaultItem.Id;
+                    }
+
+                    _terminal.WriteLine("No default option is configured.\n");
+                    continue;
+                }
+
+                if (!int.TryParse(optionStr, out int option))
+                {
+                    _terminal.WriteLine($"Invalid option '{optionStr}'. Expected a number.\n");
+                    continue;
+                }
+
+                if (_items.All(x => x.Id != option))
+                {
+                    _terminal.WriteLine($"Invalid option '{optionStr}'.\n");
+                    continue;
+                }
+
+                return option;
+            }
+        }
+
+        public IEnumerator<TerminalMenuItem> GetEnumerator()
+        {
+            return _items.GetEnumerator();
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return ((IEnumerable) _items).GetEnumerator();
+        }
+    }
+
+    public class TerminalMenuItem
+    {
+        public TerminalMenuItem(int id, string name, bool isDefault = false)
+        {
+            Id = id;
+            Name = name;
+            IsDefault = isDefault;
+        }
+
+        public int Id { get; }
+
+        public string Name { get; }
+
+        public bool IsDefault { get; }
+    }
+}