Do you want to bundle the library with the extension - or load it from a CDN at run-time?
A very simple example of a 3rd party library being bundled with an extension can be found in the IE-Check extension.
There's also several core plugins (slider, matching and media) that use 3rd party libraries, these all take advantage of support for a 'libraries' folder we added in Adapt 2.0.13 - code in the libraries folder doesn't get 'watched' for changes, making the build process a lot faster. All the core plugins are components but I think extensions also support libraries folders.
I didn't find the right solution from the internet.