- add doc comments to `AppsEndpoint`, `App` struct, and the `Get` method
in `apps.go`.
- add doc comments to the `Page` struct in `page.go`.
- add doc comments to `PagingRequest`, `PagingOption`, and paging helper
functions in `pagingRequest.go`.
- 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.