[LiveComponent] Support default value for new collection item

[IndraGunawan]

Q	A
Bug fix?	yes
New feature?	no
Deprecations?	no
Documentation?	no
Issues	Fix
License	MIT

when adding a new item to a collection of LiveCollectionType, it was always an empty value somehow, defining prototype_data is not working

so this introduces a new method to set the default value of a collection item