Feature Request: MultiSelect Select/Dropdown

I use Bootstrap / BlazorStrap quite extensively, but whenever I need a proper multi-select dropdown, I end up using another library like Radzen ([RadzenDropdown](https://blazor.radzen.com/dropdown-multiple)) or MudBlazor which aren't Bootstrap themed. I would like to see a native support in BlazorStrap. I think this would belong as an extension, since it's not necessarily a native HTML/Bootstrap feature.

I'm willing to contribute this feature if the maintainers agree on this approach.