Add skeleton loading to table summary component

[naman03malhotra]

Presently the table summary component does not have a check on the contents of the data object being passed to it. We ran into an issue with the transactions list component in WCPay client where the summary.value is being passed as undefined and it was rendered as-is. One of the ways to solve that was to pass a default value, more details here: Automattic/woocommerce-payments#1639.

However, to maintain consistency with the table component we'd like a similar skeleton loader for the TableSummary as well. Which will be rendered while the data is still being loaded.

It should look something like this

The component will take isLoading as prop which will append a class that will enable skeleton loading animation.

[htdat]

@naman03malhotra - I've done some research on this issue. I will also work on this issue too if you don't mind.