Knowledge panels: add new "action" element

[monsieurtanuki]

Is your feature request related to a problem? Please describe.
Featured in openfoodfacts/smooth-app#1136 and https://world.openfoodfacts.org/api/v2/product/4006922007543.json?fields=knowledge_panels, we need to decode a new KnowledgePanelElementType like that:

{
	"action_element": {
		"actions": ["add_categories", "add_nutrition_facts"],
		"html": "Could you add the information needed to compute the Nutri-Score?"
	},
	"element_type": "action"
}

Describe the solution you'd like
There's probably a limited list of possible values for "actions", but we'll be safer with static const String values than with enums.