Added Unified connection support (#3785)

* Added Unified connection support

* Use generic way to do expandNode.
Cleanup the ported code and removed unreference code. Added as needed later.
Resolved PR comments.

* Minor fixes and removed timer for all expanders for now. If any providers can't response, the tree node will spin and wait. We may improve later.

* Change handSessionClose to not thenable.
Added a node to OE to show error message instead of reject. So we could show partial expanded result if get any.
Resolve PR comments

* Minor fixes of PR comments

extensions/mssql/src/prompts/question.ts

@@ -0,0 +1,68 @@
+
+'use strict';
+
+import vscode = require('vscode');
+
+export class QuestionTypes {
+    public static get input(): string { return 'input'; }
+    public static get password(): string { return 'password'; }
+    public static get list(): string { return 'list'; }
+    public static get confirm(): string { return 'confirm'; }
+    public static get checkbox(): string { return 'checkbox'; }
+    public static get expand(): string { return 'expand'; }
+}
+
+// Question interface to clarify how to use the prompt feature
+// based on Bower Question format: https://github.com/bower/bower/blob/89069784bb46bfd6639b4a75e98a0d7399a8c2cb/packages/bower-logger/README.md
+export interface IQuestion {
+    // Type of question (see QuestionTypes)
+    type: string;
+    // Name of the question for disambiguation
+    name: string;
+    // Message to display to the user
+    message: string;
+    // Optional placeHolder to give more detailed information to the user
+    placeHolder?: any;
+    // Optional default value - this will be used instead of placeHolder
+    default?: any;
+    // optional set of choices to be used. Can be QuickPickItems or a simple name-value pair
+    choices?: Array<vscode.QuickPickItem | INameValueChoice>;
+    // Optional validation function that returns an error string if validation fails
+    validate?: (value: any) => string;
+    // Optional pre-prompt function. Takes in set of answers so far, and returns true if prompt should occur
+    shouldPrompt?: (answers: {[id: string]: any}) => boolean;
+    // Optional action to take on the question being answered
+    onAnswered?: (value: any) => void;
+    // Optional set of options to support matching choices.
+    matchOptions?: vscode.QuickPickOptions;
+}
+
+// Pair used to display simple choices to the user
+export interface INameValueChoice {
+    name: string;
+    value: any;
+}
+
+// Generic object that can be used to define a set of questions and handle the result
+export interface IQuestionHandler {
+    // Set of questions to be answered
+    questions: IQuestion[];
+    // Optional callback, since questions may handle themselves
+    callback?: IPromptCallback;
+}
+
+export interface IPrompter {
+    promptSingle<T>(question: IQuestion, ignoreFocusOut?: boolean): Promise<T>;
+    /**
+     * Prompts for multiple questions
+     *
+     * @returns {[questionId: string]: T} Map of question IDs to results, or undefined if
+     * the user canceled the question session
+     */
+    prompt<T>(questions: IQuestion[], ignoreFocusOut?: boolean): Promise<{[questionId: string]: any}>;
+    promptCallback(questions: IQuestion[], callback: IPromptCallback): void;
+}
+
+export interface IPromptCallback {
+    (answers: {[id: string]: any}): void;
+}