Add IProvider and IProviderCollection interfaces and implementations

This change introduces a new provider model into PowerShell Editor
Services so that feature components can have an arbitrary number of
providers registered which are used in addition to any default
implementations.  This is intended to be used as an extensibility
mechanism.

Author: daviwil

src/PowerShellEditorServices/Providers/IProvider.cs

@@ -0,0 +1,19 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+namespace Microsoft.PowerShell.EditorServices
+{
+    /// <summary>
+    /// Defines the contract for a feature provider, particularly for provider identification.
+    /// </summary>
+    public interface IProvider
+    {
+        /// <summary>
+        /// Specifies a unique identifier for a provider, typically a
+        /// fully-qualified name like "Microsoft.PowerShell.EditorServices.MyProvider"
+        /// </summary>
+        string ProviderId { get; }
+    }
+}

src/PowerShellEditorServices/Providers/IProviderCollection.cs

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+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System.Collections.Generic;
+
+namespace Microsoft.PowerShell.EditorServices
+{
+    /// <summary>
+    /// Defines the contract for a collection of provider implementations.
+    /// </summary>
+    public interface IProviderCollection<TProvider> : IEnumerable<TProvider>
+        where TProvider : IProvider
+    {
+        /// <summary>
+        /// Adds a provider to the collection.
+        /// </summary>
+        /// <param name="provider">The provider to be added.</param>
+        void Add(TProvider provider);
+    }
+}

src/PowerShellEditorServices/Providers/ProviderBase.cs

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+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+namespace Microsoft.PowerShell.EditorServices
+{
+    /// <summary>
+    /// Provides a base implementation of IProvider.
+    /// </summary>
+    public abstract class ProviderBase : IProvider
+    {
+        /// <summary>
+        /// Gets the provider class type's FullName as the
+        /// ProviderId.
+        /// </summary>
+        public string ProviderId => this.GetType().FullName;
+    }
+}

src/PowerShellEditorServices/Providers/ProviderCollection.cs

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+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+
+namespace Microsoft.PowerShell.EditorServices
+{
+    /// <summary>
+    /// Provides a default implementation of IProviderCollection.
+    /// </summary>
+    public class ProviderCollection<TProvider> : IProviderCollection<TProvider>
+        where TProvider : IProvider
+    {
+        #region Private Fields
+
+        private List<TProvider> providerList = new List<TProvider>();
+
+        #endregion
+
+        #region IProviderCollection Implementation
+
+        void IProviderCollection<TProvider>.Add(TProvider provider)
+        {
+            if (!this.providerList.Contains(provider))
+            {
+                this.providerList.Add(provider);
+            }
+        }
+
+        IEnumerator<TProvider> IEnumerable<TProvider>.GetEnumerator()
+        {
+            return this.providerList.GetEnumerator();
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return this.providerList.GetEnumerator();
+        }
+
+        #endregion
+    }
+}