Add constants for component.openForms.dataSrc values

[viktorvanwijk]

Components Select, SelectBoxes, and Radio can use another variable as a data source. In the component, this is displayed as follows:

component: Component = {
     ...
     "openForms": {
        "dataSrc": "manual",
        ...
     },
     ...
}

where dataSrc can be "manual" or "variable".

#5007 will introduce checks on this data source, and #4996 will introduce a new one: "referentielijsten".

Might be good to add a constant for this.

[viktorvanwijk]

Also, the OpenFromsConfig typed dict does not contain a dataSrc field.