Add GridFS support

[spencerjibz]

Background

Hi there, I have used gridFS for a while now, so recently thought might as well build a tus-store for it, I know there was a related issue to this back in 2016.

Implementation Details

• Extended MongoDB's gridfs write stream to support updating existing files. (can contribute this back to the mongo repo later).
• Added a config Store for usage, internally used by the store package, but can also be used separately with other stores(FileStore).
• Modified the demos to include the new store.
• Added tests for the new store and also a MongoDB-in-memory server for tests and CI
• Added e2e test for the gridfs-store

Let me know if there are any changes you'd like me to make.

[spencerjibz]

Just realised, l need to add e2e tests for the new store.

[Murderlon]

Thanks a lot for the massive contribution. Because this is a big PR, it will require a deep dive review for which I don't have time now but I'll come back to it.

[Murderlon]

Sorry for the enormous delay. I really appreciate you taking the time to contribute. After thinking about it, I don't think taking on this maintenance burden and complexity is worth it for such a niche store. However I'll happily link this in the readme as a community plugin. Would you be interested in that?