List the minimum size of a spot request sent to all dealers, so a request can be
validated before submission. Requests addressed to specific counterparties and
quotes are never subject to a minimum.

Minimums are per asset, in the asset's native denomination, matched against
terms.instrument.security (admin + id) and compared with
terms.quantities.security. An asset with no minimum cannot be sent to all
dealers (see POST /api/rfq/requests).