showing the progress of the pdf generation from callback

**Is your feature request related to a problem? Please describe.**
when working with large pdf, it takes some time to create and render the pdf, when it is happening it shows blank screen in the iframe, which is not that much good experience, loading or some kind of progress which user can see will act as good user experience

**Describe the solution you'd like**
passing a simple callback function, which will get called with latest updated value of the progress, using which progress can be shown as wanted

**Describe alternatives you've considered**
tried, adding loader untill i get blob from the lib, but loader is not that much good experience, if some kind of numbers that this much progress is complete then it will be great for user and other use cases

**Additional context**
I've created a PR(https://github.com/diegomura/react-pdf/pull/2437) where I've currently implemented the page processing progress as a callback. I'm not familiar with the entire codebase, so I've implemented this basic functionality for now. If someone can guide me, I can make further improvements. I haven't implemented any tests yet.