Flesh out CanonicalMetadata about cs3apis

[labkode]

Execution bit can be added to CanonicalMetadata in:
https://cs3org.github.io/cs3apis/#cs3.storage.provider.v1beta1.CanonicalMetadata

Let's start with this use-case first and tackle MacOS labels and others later.

A new field needs to be defined in the WebDAV layer so clients (desktop mainly) can understand this new capability and set local permissions accordingly:

Code 	Resource 	Description
S 	File or Folder 	is shared
R 	File or Folder 	can share (includes reshare)
M 	File or Folder 	is mounted (like on DropBox, Samba, etc.)
W 	File 	can write file
C 	Folder 	can create file in folder
K 	Folder 	can create folder (mkdir)
D 	File or Folder 	can delete file or folder
N 	File or Folder 	can rename file or folder
V 	File or Folder 	can move file or folder