feat(search-box): add loading indicator + `showLoadingIndicator` prop

[aymeric-giraudet]

Summary

Work Item : FX-377

To stay as close as InstantSearch.js's interface as possible, we decided to add a showLoadingIndicator prop which is true by default. The only difference is that we are not able to provide a custom component, which is not feasible for submit/reset buttons anyway.

Result

By default, when search is stalled, the reset button will be hidden, and a loading indicator is shown. If showLoadingIndicator is set to false, it will never show a loading indicator, and the reset button will still be present if search is stalled.

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