Fix/playground virtualisation loader

[andreas-unleash]

Fixes:

• Displays loader when api is loading
• Use VirtualisedTable (list virtualisation) for the results

About the changes

Closes # .

Relates to https://github.com/Unleash/unleash/issues/1873

Important files

Discussion points

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Name	Status	Preview	Updated
unleash-frontend	✅ Ready (Inspect)	Visit Preview	Aug 10, 2022 at 0:33AM (UTC)

[FredrikOseberg]

Could we update the description here and add the issue reference?