Submit `sepa_country_interest` as metadata from the form at /fr/contactez-nous

For `ProspectAction`s in GoCardless, we have the concept of "metadata" fields. `sepa_country_interest` fits that category becuase it is not relevant for all kinds of prospect.

This pull request updates the form at `/fr/contactez-nous` to submit `sepa_country_interest` as metadata.