Initial Marktplaats SDK scaffold

Clone the twdehands SDK into mrktplaats and retarget naming and defaults for app.marktplaats.nl while keeping the same request/response bodies and endpoint behavior.

relevant.go

@@ -0,0 +1,47 @@
+package mrktplaats
+
+import (
+	"context"
+	"fmt"
+	"net/url"
+)
+
+// RelevantService retrieves similar/related listings.
+type RelevantService struct {
+	t *transport
+}
+
+// SimilarAdCore is the ad core from the older API3 format (snake_case).
+type SimilarAdCore struct {
+	ID      string           `json:"id"`
+	Title   string           `json:"title"`
+	Picture DiscoveryPicture `json:"picture"`
+	Price   DiscoveryPrice   `json:"price"`
+}
+
+// SimilarItem is a related listing.
+type SimilarItem struct {
+	AdCore       SimilarAdCore `json:"ad_core"`
+	PageLocation string        `json:"page_location"`
+}
+
+// SimilarAdsResponse contains similar listings.
+type SimilarAdsResponse struct {
+	Items      []SimilarItem `json:"items"`
+	ResultType string        `json:"result_type"`
+}
+
+// Get returns similar/related listings for a given listing.
+func (s *RelevantService) Get(ctx context.Context, listingID string, sellerID string, categoryID int) (*SimilarAdsResponse, error) {
+	path := fmt.Sprintf("/api3/ads/relevant/%s.json", listingID)
+	params := url.Values{}
+	if sellerID != "" {
+		params.Set("seller_id", sellerID)
+	}
+	params.Set("category_id", fmt.Sprintf("%d", categoryID))
+	var resp SimilarAdsResponse
+	if err := s.t.get(ctx, path, params, &resp); err != nil {
+		return nil, err
+	}
+	return &resp, nil
+}