Extend InitiateFileUpload to support exclusive uploads and etags-based checks

[glpatcern]

This is a revamp of #87, which got closed because the master branch got renamed to main...

Pasting here the relevant parts:

This extension is motivated by cs3org/wopiserver#25. Discussing with @ishank011 it looks reasonable to extend the InitiateFileUploadRequest to support exclusive operations: I may want to upload a file "no matter what", overwriting the destination - which is the current behavior. Or I may want to atomically check-and-upload, which is the scope of this new flag. Both actions are legitimate and should be supported.

The PR proposes a minimal extension of the protocol. May we need to support other POSIX(-like) flags and have rather an enum instead of just a bool? I guess by now we're settled, yet for reference the years-old open() call has plenty of options.

Related to this, there's been quite some discussion around locking within ownCloud, that's why I'm involving @butonic too.

Comments/opinions welcome of course, but I'd like to move forward so to start testing the WOPI server locking logic with oCIS.

[glpatcern]

Following discussions, a quick summary. We should follow the if-match etags preconditions used in HTTP headers and implement:

• A more explicit upload-if-absent flag (rename exclusive)
• A upload-if-match option to handle the etag use-case from sync clients

We agree that the default behavior without options is to overwrite the destination without further checks. Name of the optional arguments to be agreed.

Also, the response MUST contain the current etag when upload-if-match is used and the precondition fails.

[glpatcern]

Reviewing myself, two comments:

1. Shall we use a oneof grup for if_not_exist and if_match, i.e. make them alternative? I'm using a oneof for both options, please have a look
2. In case of etag mismatch, do we want the current etag to be returned as opaque or do we give a specific (again optional) field in the InitiateFileUploadResponse?