Add-ons support

[keianhzo]

This PR adds support for Add-ons in Hubs. For some background, see here: #6099

• You can find the Hubs TS typings here (WIP): https://github.com/MozillaReality/hubs-ts-types

• A template to simplify add-on development: https://github.com/MozillaReality/hubs-template-addon

There are also a couple demo add-on:

• Duck add-on. This was previously part of the client code but in this branch has been converted to an add-on: https://github.com/MozillaReality/hubs-duck-addon

• Portals add-on. A simple portals implementation as an add-on to illustrate a more complex Hubs add-on:
  https://github.com/MozillaReality/hubs-portals-addon

To add add-ons to a client:

• Install the add-on node module.
• Add your add-on module to the addons.json file.

{
  "addons": [
    "hubs-duck-addon", 
    "hubs-portals-addon"
  ]
}

• Create a hubs room
• Enable the add-on in the room settings

• The add-on would run in the Hubs client after the client reloads.