fix(hp): Make the updated hanging protocol compatible with modes (#2971)

There had been a change made to make the hanging protocols
auto initialize, but that caused problems on mode init
because the couldn't specify the hanging protocols any longer.
THis change restores that, and also improves the API a little
bit by allowing the display set selectors to be specified by id
instead of positionally.

fix(hp):PR Review Comments

fix(hp):Unit test and docs

platform/core/src/extensions/ExtensionManager.js

@@ -287,7 +287,10 @@ export default class ExtensionManager {
   _initHangingProtocolsModule = (extensionModule, extensionId) => {
     const { HangingProtocolService } = this._servicesManager.services;
     extensionModule.forEach(({ id, protocol }) => {
-      HangingProtocolService.addProtocol(id, protocol);
+      if (protocol) {
+        // Only auto-register if protocol specified, otherwise let mode register
+        HangingProtocolService.addProtocol(id, protocol);
+      }
     });
   };