Add indicator when product analytics panels are loading slow

What does this MR do and why?

Sometimes a CubeJS query can take a long time to execute. In this scenario, the CubeJS client receives a continue wait response from the server to indicate that the query is still running.

In this situation, we now show an additional loading state on the dashboard panel to give the user some feedback that things are still running.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

Screen recording (with faked / hardcoded timings!)

How to set up and validate locally

1. Follow these instructions to setup Product Analytics in GDK.
2. Onboard Product Analytics by setting up the feature at Project > Analytics > Analytics dashboards.
3. Generate some events for this project so your dashboards have data.
4. View a dashboard, and try trigger the slow loading / "continue wait" scenario by:

mocking the Cube response

In Chrome browser:

• Open chrome devtools
• find one of the Cube network requests, /load
• right click on it, click override content
• paste in the continue wait response: {"error":"Continue wait"}
• refresh the dashboard, every response will now be continue wait
• note: this might slow down your PC, as the CubeJS client will start spamming out new requests in response to each continue-wait, basically causing an infinite loop. I suggest closing the tab when you're not actively looking at it 😅
• remember to clean up the override when you're done!

or

Causing the wait for realsies

I had trouble reproducing this continue wait response consistently in my local dev env, but managed to get it occasionally by:

• seeded my local clickhouse with ~8 million events for this project
• reduced available memory for cube
  • open analytics-devkit/docker-compose.yml
  • add mem_limit: 128m to the cube section
  • I tried lowering it from 128m but cube was then frequently crashing, not sure what the sweet-spot minimum is...
• now run queries that span larger timeframes
• I was seeing continue wait maybe 1 in 10 times I loaded the dashboard

Related to #420325 (closed)