make dataprovider send headers

[butonic]

Instead of ocdav doing an additional Stat request we now rely on the dataprovider to return the necessary metadata information as headers. As a bonus it prevents race conditions between the Stat and InitiateFileDownload calls.

related: owncloud/ocis#4153

[labkode]

@butonic what about encapsulating all this information into a JSON structure and pass that payload into one single header that can be decoded in a simpler way compared to have the information spread across multiple headers?

[butonic]

@labkode most of the headers are standard headers: ContentType, ContentDisposition, ETag and LastModified.

Which leaves the OC prefixed headers: OC-ETag (which in contrast to ETag remains the same, regardless of the content encoding, so clients can use it to sync), OC-FileId and OC-Checksum.

• OC-Checksum should be deprecated in favor of Digest

• For the other headers maybe sth like the TUS Upload-Metadata could be used. It could be Download-Metadata, CS3-Metadata or individual CS3-{property} headers.

The next question then becomes how can clients request what metadata to return, eg by providing a field mask in the get request ... again, as a header.

[butonic]

OC-FileId should be deprecated and split into CS3-StorageID, CS3-SpaceID and CS3-OpaqueID,
OC-ETag should be deprecated in favor of a CS3-ETag

related owncloud/ocis#3782