[Search] Make CustomComponents more adjustable

[artmarks]

Issue description

We need a specific info message if there is no result found in the search. The message should have a certain format and coloring.

Discussion

To realize this we could create a new optional prop customComponents that looks like this:

export interface CustomComponents {
    Menu?: ComponentType<MenuProps<SearchOption, false, SearchGroupOption>> | undefined,
    Input?: ComponentType<InputProps<SearchOption, false, SearchGroupOption>> | undefined,
    SingleValue?: ComponentType<SingleValueProps<SearchOption, false, SearchGroupOption>> | undefined,
    Option?: ComponentType<OptionProps<SearchOption, false, SearchGroupOption>> | undefined,
    NoOptionsMessage?: ComponentType<NoticeProps<SearchOption, false, SearchGroupOption>> | undefined,
    LoadingMessage?: ComponentType<NoticeProps<SearchOption, false, SearchGroupOption>> | undefined,
    ValueContainer?: ComponentType<ValueContainerProps<SearchOption, false, SearchGroupOption>> | undefined,
    IndicatorsContainer?: ComponentType<IndicatorsContainerProps<SearchOption, false, SearchGroupOption>> | undefined,
    ClearIndicator?: ComponentType<ClearIndicatorProps<SearchOption, false, SearchGroupOption>> | undefined
}

and exchange thoose given components with the defaults ones in useCustomComponents.

We would also be open to other suggestions to our issue :)

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[mbeckem]

I wouldn't go as far as exposing all possible options that react-select gives us. I'd like to preserve the option to switch to another component in the future. Exposing an API that is very close to react-select's functionality would make that very hard.
The following features may be a good compromise for the time being (with PRs being highly welcome):

• A prop to configure an optional noResultsMessage (a string or even a custom react component). I don't know at this point if there is any context needed to render this message (there are two states: never searched and thus no results, i.e. "fresh" widget; and a search was attempted but returned no results). These may even be two props now that I think about it.
• A prop to configure a custom loading message. You could, just as an idea, even be very fancy here: e.g. render which search sources are still searching and which are complete.

I think other 'overrides' are a bit out of scope at the moment and would need a good justification before thinking about implementing them.