fix: Send factory as string in SchemaWidget and filter field types in types control panel

[Shyam-Raghuwanshi]

When adding a new field in the dexterity types schema editor (/controlpanel/dexterity-types//schema), the field factory was sent to the backend as an object (e.g. {value: "label_text_field", label: "Text"}) instead of a plain string, causing a "missing parameter factory" error. Additionally, form-only field types (like static_text, hidden, radio_group) were shown in the field type dropdown even though they don't have corresponding Plone field factories on the backend.

Ref: plone/plone.restapi#1969

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