Emit filter input clear event from GlFilteredSearch
# Summary
Currently `GlFilteredSearch` component only emits `submit` event when user has some token values provided and either clicks `Search` button, or hits <kbd>Enter</kbd>/<kbd>Return</kbd> key while cursor is in input field. However, `submit` event is not emitted when user clears the filtered input field, this is specifically a requirements in case of filtered search as it is a more advanced component (compared to plain input field with clear button) as any page that has data filtered based on filters provided, clearing the input needs to refetch the unfiltered data. First use-case of this feature is in Requirements page where we're using `GlFilteredSearch` (see https://gitlab.com/gitlab-org/gitlab/-/merge_requests/32961 & https://gitlab.com/gitlab-org/gitlab/-/merge_requests/33484).
We can make the component either emit `submit` event (with empty value) or `clear` event in case user clicks on `Clear` button to clear the filter values.
issue