Make ElementPickerValueConverter public, consistent with other property value converters

[glombek]

ElementPickerValueConverter is internal sealed, from 18.0.0 through 19.0.0-beta1. It's the only internal converter in Umbraco.Core and Umbraco.Infrastructure. Every other built-in property value converter is public, including the new 19 pickers such as MultipleDocumentPickerValueConverter.

Because it's internal, packages can't:

• remove or replace it with the documented builder.PropertyValueConverters().Remove<ElementPickerValueConverter>()
• take it as a dependency to wrap or extend it

That's how packages like SuperValueConverters return Models Builder types for pickers. For example, a picker that only allows MyElement returns IEnumerable<MyElement>, not IEnumerable<IPublishedElement>. With the Element Picker, that currently needs reflection.

Was it made internal on purpose? If not, could it be made public, ideally before 19.0.0 is released?

[AndyButland]

Thanks for flagging this @glombek. We've addressed in #24569, so it'll be part of the first release candidate.