Upload files resumably, using tus

[gnprice]

The server recently added (and Zulip web started using) an API feature to support resumable file uploads using the "tus" protocol. Resumable uploads enable uploading larger files, and better handling failures in the middle of an upload.

This would be a useful feature to take advantage of on mobile too.

For details, see:

• Large files fail to upload zulip#9391
• Chunk upload support using tusd service. zulip#26004
• Server API docs: tus: Document new API level for TUS endpoints. zulip#31682
• Protocol docs for tus: https://tus.io/protocols/resumable-upload

The web implementation is pretty simple because it uses the NPM package @uppy/tus; that one in turn wraps tus-js-client. A quick web search finds several tus client implementations for Dart, so hopefully one of those is good and we can easily just use it.