Collection Creator Usability: Warn on mixed datatype collections

[jmchilton]

Like empty collections, there are times you might want to have a mixed datatype collection but 99+% percent of the time I think collections should be uniformly datatyped. It would be nice if the collection creators issued a little warning if the datatypes don't match along the lines of the excellent work of @ahmedhamidawan in #19250.

There may be a few more usecases for mixed dbkey collections but it would be great if there was some sort of lighter weight indicator about that too. Maybe the thing for both dbkey and extension is that they should be highlighted in the UI if they are mixed across the collection - so right now we show the extension for every element in a list - maybe only do that if they are different with a little warning next to them and then match that for dbkey?