[select] Add loading prop for async option fetching

Summary

When fetching options asynchronously for <Select>, there is currently no built-in way to indicate a loading state (e.g., disable the select, show a spinner, or display a loading option). This is especially important for dynamic forms where options are loaded from an API.

Examples

<Select value={value} disabled={isLoading}>
  {isLoading ? (
    <MenuItem disabled>Loading...</MenuItem>
  ) : (
    options.map(option => <MenuItem key={option} value={option}>{option}</MenuItem>)
  )}
</Select>

Motivation

A dedicated loading prop would allow for a more consistent and accessible UX, similar to what exists for <Autocomplete>.

Search keywords: select, async