Add the file endpoint to download documents

[aschempp]

Lexware Office has deprecated the document entpoint, see e.g. https://developers.lexware.io/docs/#delivery-notes-endpoint-render-a-delivery-note-document-pdf

This adds a new FileClientTrait and transparently maps the existing calls to the new endpoint.

[Sysix]

Thank you for the PR @aschempp

• I guess the Clients\File should be deprecated too
• Does the Trait need to be deprecated? Only the implementation is changed, not the interface
• Fixing CI

If you do not have time for the suggestion, I will try on the weekend to patch this :)

[aschempp]

• I guess the Clients\File should be deprecated too

The files endpoint (https://developers.lexware.io/docs/#files-endpoint) does not seem to be deprecated, but still used to upload files (e.g. parse invoices) as far as I understand. I'm not using that endpoint myself though, so I'm not really familiar with it.

• Does the Trait need to be deprecated? Only the implementation is changed, not the interface

The /documents entpoint has been deprecated everywhere, so I would suggest to remove that trait in the next major version and only keep the /file endpoint. Or maybe I'm misunderstanding?

• Fixing CI

Sorry about that, should be fixed in 7d558a9

[Sysix]

Thank you! Will add some commits on top before releasing a new version