Added api/classifications endpoint, requiring a new query

internal/api/opportunities_test.go

@@ -42,6 +42,15 @@ func opportunitiesServer(t *testing.T, count int, opts ...Option) string {
 			UUIDType: "ocid",
 			Title:    fmt.Sprintf("Opportunity %d", i),
 			Source:   "FindATender",
+			// Two codes, shared: ocds-1, ocds-3... have 72000000, the
+			// others 03000000.
+			Classifications: []types.ClassificationCode{
+				{
+					IsCPV:          true,
+					Code:           []string{"03000000", "72000000"}[i%2],
+					Classification: []string{"Agriculture", "IT services"}[i%2],
+				},
+			},
 			Releases: []types.OpportunityRelease{
 				{
 					ReleaseID:   "r1",