Create MS.SqlTools.Credentials project (#249)

* Initial credential service files

* Clean-up hostloader

* Update build scripts to build credentials archive

* Move hosting files to new assembly

* Add credentials files to MS.SqlTools.Credentials

* Remove duplicate files

* Update namespace in program.cs

* Fix test build breaks

* Update extensions visibility.

* Remove unused resource strings

* Add xproj files to SLN for appveyor builds

* Fix appveyor build break in test project

* Fix extensibility tests

* Fix various typos in latest iteration

* Add settings for Integration build

* Fix codecoverage.bat to use full pdb for new projects

* Fix bug when packing in folder with native images

* Fix typos in xproj

* Reset XLF to fix build.cmd

src/Microsoft.SqlTools.Credentials/Credentials/ICredentialStore.cs

@@ -0,0 +1,41 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using Microsoft.SqlTools.ServiceLayer.Credentials.Contracts;
+
+namespace Microsoft.SqlTools.ServiceLayer.Credentials
+{
+    /// <summary>
+    /// An <see cref="ICredentialStore"/> support securely saving and retrieving passwords
+    /// </summary>
+    public interface ICredentialStore
+    {
+        /// <summary>
+        /// Saves a Password linked to a given Credential
+        /// </summary>
+        /// <param name="credential">
+        /// A <see cref="Credential"/> to be saved. 
+        /// <see cref="Credential.CredentialId"/> and <see cref="Credential.Password"/> are required
+        /// </param>
+        /// <returns>True if successful, false otherwise</returns>
+        bool Save(Credential credential);
+
+        /// <summary>
+        /// Gets a Password and sets it into a <see cref="Credential"/> object
+        /// </summary>
+        /// <param name="credentialId">The name of the credential to find the password for. This is required</param>
+        /// <param name="password">Out value</param>
+        /// <returns>true if password was found, false otherwise</returns>
+        bool TryGetPassword(string credentialId, out string password);
+
+        /// <summary>
+        /// Deletes a password linked to a given credential
+        /// </summary>
+        /// <param name="credentialId">The name of the credential to find the password for. This is required</param>
+        /// <returns>True if password existed and was deleted, false otherwise</returns>
+        bool DeletePassword(string credentialId);
+
+    }
+}