Migrate FormVariable's attribute field into prefill_options

[vaszig]

This can be done after #4620 has been merged (This PR adds the prefill_options field to the model).

The attribute should be part of the prefill_options JSON field and the general/ideal structure should be:

• prefill_plugin -> determines the shape of prefill_options
• prefill_options -> stores plugin-specific configuration options (will contain the attribute key for all the other plugins)
• prefill_identifier_role -> still used to point to which BSN/KVK to grab from the Submission for auth checks etc.

[joeribekker]

Might be backwards incompatible with custom plugins (read: NHR, DH) and streamlines plugin architecture.

[joeribekker]

This is a prelude to moving the prefill config from the component to the variables tab.