[Feature] SingleSelect & MultiSelect: Add onChangeWithoutDebounce()

[riitasointi]

Description

This PR adds a new prop called onChangeWithoutDebounce() to <SingleSelect> & <MultiSelect>. The prop is basically the same as onChange() but does not use the debounce time even if it is specified.

Its inteded use case is mainly enabling the loading state of the component. It makes sense to show the loading spinner immediately when the user starts filtering and only make a backend call after a certain debounce time.

In addition, this PR also fixes an oversight in the SingleSelect component getDerivedStateFromProps() logic. Previously, when the value of the items prop or the selectedItem prop changed, the filter input's value was always set to an empty string ''. Now it only does that if selectedItem is set to null.

Motivation and Context

It was reported by users that having the loading state of the component only be applied after the debounce time is not ideal. It causes the existing items to be filtered briefly by the new input before the loading process begins.

How Has This Been Tested?

Styleguidist on Chrome & library's programmatic tests

Release notes

SingleSelect

• Fix filter input value parsing when items or selectedItem prop changes
• Add onChangeWithoutDebounce() prop. Inteded to be used to set the loadingprop

MultiSelect

• Add onChangeWithoutDebounce() prop. Inteded to be used to set the loadingprop