[companion] tus uploads are always deferred

[dschmidt]

Initial checklist

• I understand this is a bug report and questions should be posted in the Community Forum
• I searched issues and couldn’t find anything (or linked relevant results below)

Link to runnable example

No response

Steps to reproduce

Use companion to upload to a tus server without support for the https://tus.io/protocols/resumable-upload#upload-defer-length header (afaict implementing support for the header is optional).
ownCloud Infinite Scale for example does not implement it, c.f. owncloud/ocis#10794

It broke in #4697

Expected behavior

Upload works.

Actual behavior

It does not work and there does not seem to be a way to disable deferred length uploads.

[dschmidt]

If the Server supports deferring length, it MUST add creation-defer-length to the Tus-Extension header.

For oCIS it looks like this:

http --verify no OPTIONS https://host.docker.internal:9200/remote.php/dav/spaces/storage-users-1$some-admin-user-id-0000-000000000000
[…]
Tus-Extension: creation,creation-with-upload,checksum,expiration
[…]

The header is also available from error.originalRequest in the onError hook, so it would also be possible to detect the likely reason for the failure there, which seems to be a bit of a hacky way.

I'm not sure what the best approach is:
We could either try to detect upfront whether the extension is supported, so users don't need to know about this.
We could always assume it's supported and check after a failed request if the extension is missing and then retry without it...
Or we explicitly enable/disable it via Companion configuration as in my pull request, which saves some requests but offers more configuration surface and requires more knowledge from the user...
Or we introduce a tristate option: defer: yes/no/auto (the latter being an additional options request)