Restrict BlockItemType for ContentType as in Pages and Posts

[ClausRogisch]

It would be nice, for Content implementing IBlockContent, to be able to restrict the usable Blocks, as is implemented for Pages and Posts via the BlockItemTypeAttribute

In the ContentTypeBuilder, possibility to adapt code for PageType to ContentType (and maybe also for ContentGroup)
`
// Add block types
var blockTypes = type.GetCustomAttributes();
foreach (var blockType in blockTypes)
{
pageType.BlockItemTypes.Add(blockType.Type.FullName);
}

return pageType;
`

[ClausRogisch]

#1889 made the changes to use [BlockItemTypeAttribute] on [ContentType] to restrict the available Blocks in the Manager when adding Blocks to the ContentType.