Add support for label/fieldset in serializer

Important for easly create a kind of FormView from Form ressource in a SDK (outside of SF2)

render for exemple :
```xml
<form>
    <label for="form_name">custom label</label>
    <input type="text" id="form_name" name="form[name]" required="required"/>
    <textarea id="form_description" name="form[description]" required="required">Desc</textarea>
    <label for="form_gender">custom</label>
    <select name="form[gender]" required="required" id="form_gender">
        <option value="m">male</option>
        <option value="f">female</option>
    </select>
    <label for="form_genderRadio">label</label>
    <fieldset id="form_genderRadio">
        <label for="form_genderRadio_0">male</label>
        <input type="radio" id="form_genderRadio_0" value="m" name="form[genderRadio]" required="required"/>
        <label for="form_genderRadio_1">female</label>
        <input type="radio" id="form_genderRadio_1" value="f" name="form[genderRadio]" required="required"/>
    </fieldset>
    <input type="hidden" id="form_limit" name="form[limit]"/>
    <input type="email" id="form_towns_par" value="Paris" name="form[towns][par]" class="email-box"/>
    <input type="email" id="form_towns_lon" value="London" name="form[towns][lon]" class="email-box"/>
    <label for="form_public">label</label>
    <input type="checkbox" id="form_public" value="1" checked="1" name="form[public]"/>
</form>
```