Add new embed event `availabilityLoaded` when all the availability for a booking page has loaded in embed

Look at `bookingSuccesfulV2` event on how it is fired and let's fire it in the similar way.

Goal is to be inform the embedding page when the slots or availability have been loaded. Event data for now could be 
`eventId`, `eventSlug`, `eventTitle` and we could add more data as needed later.