Adding error message and placeholder for dropdown (#17012)

* Adding error message and placeholder for dropdown

* Fixing default error message for empty required dropdown

* Fixing a small input value updating bug

* Fixed a commend

* Fixed a comment

* PR fixes

* Using codicon

* Fixed screen reader not reading error message

* Adding support for multiple dropdowns error message

* Adding error border

* Fixing error border styling.

* Added more specific classes for styling

* Making styling more specific

src/sql/azdata.proposed.d.ts

@@ -430,6 +430,17 @@ declare module 'azdata' {
 		enabled?: boolean;
 	}
 
+	export interface DropDownProperties {
+		/**
+		 * Adds a short hint that describes the expected value for the editable dropdown
+		 */
+		placeholder?: string;
+		/**
+		 * Define error messages to show when custom validation fails. Note: For empty required dropdowns we use a default error message.
+		 */
+		validationErrorMessages?: string[];
+	}
+
 	/**
 	 * Panel component with tabs
 	 */