[Feature] Add onLoadingSlow interceptor

[MattCCC]

The idea is to add two options into the plugin

The onLoadingSlow (null by default) feature is a user-friendly addition that helps monitor slow network conditions or when the server is taking longer than expected to respond. This callback function triggers when a request takes longer than a specified threshold (typically shorter than the timeout limit) but hasn't yet timed out. This provides an opportunity to show a loading indicator, display a message, or offer a cancel option to the user.