there is no visual indicator when running long queries on demand

[pieh]

Description

As reported in #27620 (comment)

halts in navigation while browsing localhost without visual feedback can be jarring

With query on demand Gatsby will run the queries when user tries to navigate to a page, in case of heavier queries user doesn't get the that feedback that something is happening in background that browser is waiting for. Experience is just not great in cases like this.

For now with this feature being opt-in it's not blocking because user has explicitly decide to use it, so they can be aware of why this happen, but we do need a way to indicate that navigation (or first render) is not "stalled" or broken and that runtime is waiting for something