fix: correct paging parameter logic and add paging options

- fix bug where `pageNumber` and `pageSize` were swapped during default
initialization in `apps.go`.
- fix bug in `pagingRequest.go` where `ToParams` mapped keys to the
incorrect struct fields.
- add `ForPageNumber` and `WithPageSize` functional options to allow
custom paging configuration.
- enhance `Apps` endpoint tests to verify that `pageNumber` and
`pageSize` query parameters are correctly passed to the API.
- add test case for verifying non-default paging options.

pagingRequest.go

@@ -3,15 +3,27 @@ package onspring
 import "strconv"
 
 type PagingRequest struct {
-	pageSize   int
 	pageNumber int
+	pageSize   int
 }
 
 func (pr *PagingRequest) ToParams() map[string]string {
 	return map[string]string{
-		"pageSize":   strconv.Itoa(pr.pageNumber),
-		"pageNumber": strconv.Itoa(pr.pageSize),
+		"pageNumber": strconv.Itoa(pr.pageNumber),
+		"pageSize":   strconv.Itoa(pr.pageSize),
 	}
 }
 
 type PagingOption func(*PagingRequest)
+
+func ForPageNumber(pageNumber int) PagingOption {
+	return func(pr *PagingRequest) {
+		pr.pageNumber = pageNumber
+	}
+}
+
+func WithPageSize(pageSize int) PagingOption {
+	return func(pr *PagingRequest) {
+		pr.pageSize = pageSize
+	}
+}