Add minimum quantity defaults with validation

Objective

See the objective outlined in the parent epic [Unify purchase flows] SaaS/Self-Managed Premiu... (&18567).

Purpose

This issue is to add default quantity logic for SaaS subscription based for the namespace and add validation for minimum quantity for SaaS and Self-Managed subscriptions:

• Pass seats_in_use in the purchase data for namespace
• Default quantity input with seats in use for the namespace only for SaaS subscriptions
  • When seats in use doesn't exist (for eg. SM, Compute Minutes, Storage), use default quantity value to default the quantity field
    • namespace.seats_in_use || product_rate_plan_charge.default_quantity
• Add validation for minimum quantity requirements
• Do not default quantity for any other purchase flows, eg. Self-Managed subscriptions, Compute Minutes, etc.