POST body for /v1/stays/search.

Occupancy can be specified in two ways:

• Explicit rooms — supply rooms with per-room adult counts and child
  ages. When rooms is provided it takes precedence over passengers.
• Simple traveler count — omit rooms and supply passengers
  (or rely on its default of one adult). build_stay_search_params then
  derives a single room from passengers.adults. The shorthand cannot
  carry child ages, so passengers.children > 0 without rooms is
  rejected with a field-level 422 on rooms[].childrenAges; child ages are never
  invented.

Destination can be specified in one of two mutually exclusive ways:

• Place destination — supply destination (a place identifier such
  as "milan-IT"). This is the standard city/region search.
• Property destination — supply property (a
  :class:StayPropertyDestination object from the autocomplete). Mercury
  resolves the property's city to a place identifier, searches that city,
  and streams only the solution matching the exact propertyId.

Exactly one of destination or property must be provided; supplying
both or neither returns 422 UNPROCESSABLE_ENTITY with a field-level
error.