webdav: add size to FileSystem.Create()

[DeepDiver1975]

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In some cases the total size is necessary upfront to perform the upload to an external system.

And even if not it is good to have it as a minimal test to see if data was transmitted completely ....

Open question: how to address other valuable headers like checksums or what ever ....

maybe we pass the full request object to Create()? Or body + headers?

@emersion any objections/idea?

[emersion]

Hm right… I think the use-case is reasonable, but backends should keep in mind that the size is just a hint, it may be unset (-1). Also, HTTP compression (Content-Encoding) will always leave Content-Length unset.