[EAK-523] Improve the handling of multifields inside prefixed fieldsets

[liubou-masiuk]

Example code

@FieldSet(namePrefix = "node1_")
    private Fieldset fieldset;

    @FieldSet(namePrefix = "node2/")
    private Fieldset fieldset2;

    public static class Fieldset{
        @DialogField
        @TextField
        private String fieldsetTitle;

        @DialogField
        @MultiField(value = MultifieldItem.class)
        List<MultifieldItem> items;
    }
    
    public static class MultifieldItem {
        @DialogField
        @TextField
        private String title;
        @DialogField
        @TextField
        private String description;
    }

Actual result: Currently, the fieldset prefix is added to both the multifield node name and multifield field's name. In the example above the title field will be stored in
node1_items/item0/node1_title
node2/items/item0/node2/title
respectively. It is difficult to retrieve multifield items from such nodes afterwards because the miltifield injectors are not aware about field prefixes

Expected result: the prefix is added only to top-level fields:
node1_items/item0/title
node2/items/item0/title