Support option child element for abp-select tag

[acjh]

Resolves https://stackoverflow.com/questions/67172053/empty-items-for-abp-select

Changes:

1. Set selectTagHelperOutput.Content to childContent from output in AbpSelectTagHelperService.GetSelectTagAsync:

  var selectTagHelperOutput = await selectTagHelper.ProcessAndGetOutputAsync(GetInputAttributes(context, output), context, "select", TagMode.StartTagAndEndTag);

+ selectTagHelperOutput.Content.SetHtmlContent(childContent);

2. Add [OutputElementHint("select")] on AbpSelectTagHelper so that <option> child element is allowed at design time.
   Otherwise, we get the warning Element 'option' cannot be nested inside element 'div'. from Razor in .cshtml files.
   You may need to restart Visual Studio for this to take effect.

3. Update the demo in FormElements.cshtml for https://bootstrap-taghelpers.abp.io/Components/FormElements.
   The demo is linked on https://docs.abp.io/en/abp/latest/UI/AspNetCore/Tag-Helpers/Form-elements.

Reference: https://docs.microsoft.com/en-us/aspnet/core/mvc/views/tag-helpers/authoring?view=aspnetcore-5.0#inspect-and-retrieve-child-content