[FhirResourceTree] Slice item creation

[BPierrick]

Problem

As for now, it is not possible for the user to create a slice item to an array tree node.

Description

We need to add the UI allowing the user to choose between a normal item or one of the available slices for an array node.

Additional context

• Available slices have to be present in the node definition
• Slice item creation has to respect cardinality constraint (we cannot create more slices than what is defined in its definition max attribute => This is to be handled in #613