add upload length to InitiateFileUpload

[butonic]

allows the server to allocate files or check available quota.
Needed for tus creation extension, which always expects a Upload-Length.

then replace Opaque data in

• https://github.com/cs3org/reva/pull/674/files/c79ca8c607207b3564eb61976470e3f8c198328b#diff-5fec0456a6ea9fb1227335fc8d3f8cfdR85-R92
• https://github.com/cs3org/reva/pull/674/files/c79ca8c607207b3564eb61976470e3f8c198328b#diff-1d23ba8815a000b1764e61a113e56218R254-R262
• https://github.com/cs3org/reva/pull/674/files/c79ca8c607207b3564eb61976470e3f8c198328b#diff-1de8ce7b59fd3799ac2f7c8e3a020b6dR204-R211
• https://github.com/cs3org/reva/pull/674/files/c79ca8c607207b3564eb61976470e3f8c198328b#diff-b7927c94825771f6aa388ba4bfb89ca9R121-R128

We should make this optional to allow the clients to initialize an upload using tus and the creation-defer-length extension. Using 0 to indicate creation-defer-length does not work, because you might want to create an empty file.

[PVince81]

also need the mtime to be able to set initial mtime

maybe allow arbitrary metadata ?