[api-minor] Add support for OpenAction destinations (issue 10332)

Note that the OpenAction dictionary may contain other information besides just a destination array, e.g. instructions for auto-printing[1].
Given first of all that an arbitrary `Dict` cannot be sent from the Worker (since cloning would fail), and second of all that the data obviously needs to be validated, this patch purposely only adds support for fetching a destination from the OpenAction entry[2].

---
[1] This information is, currently in PDF.js, being included through the `getJavaScript` API method.

[2] This significantly reduces the complexity of the implementation, which seems fine for now. If there's ever need for other kinds of OpenAction to be fetched, additional API methods could/should be implemented as necessary (could e.g. follow the `getOpenActionWhatever` naming scheme).

src/display/api.js

@@ -650,6 +650,14 @@ class PDFDocumentProxy {
     return this._transport.getPageMode();
   }
 
+  /**
+   * @return {Promise} A promise that is resolved with an {Array} containing the
+   *   destination, or `null` when no open action is present in the PDF file.
+   */
+  getOpenActionDestination() {
+    return this._transport.getOpenActionDestination();
+  }
+
   /**
    * @return {Promise} A promise that is resolved with a lookup table for
    *   mapping named attachments to their content.
@@ -2152,6 +2160,11 @@ class WorkerTransport {
     return this.messageHandler.sendWithPromise('GetPageMode', null);
   }
 
+  getOpenActionDestination() {
+    return this.messageHandler.sendWithPromise('getOpenActionDestination',
+                                               null);
+  }
+
   getAttachments() {
     return this.messageHandler.sendWithPromise('GetAttachments', null);
   }