Request checksum of file via endpoint

[tobiasKaminsky]

Coming from nextcloud/android#7894 (comment)

On Android an user wants to upload a file

• client detects that file with same name already exists
• we do not know if it is the same file
• client requests only for this file via new(?) endpoint a checksum
• client compares it
• only if this differs, it presents conflict resolution dialog

[szaimen]

Lets track this in #11138

[tobiasKaminsky]

Lets track this in #11138

That is different. This one is for "on-demand" checks.
If you want to combine it, then please also c&p this scenario to the other issue.

[szaimen]

Done 👍
(Added to the description of the linked issue)