[WIP] OCS API for server-to-server sharing

OCS API for server2server sharing as part of #12285 .

This is work in progress, I just want to allow people to look at it early.

This is how the API will look like:

POST to create new server2server share
```
http://localhost/oc/ocs/v1.php/apps/files_sharing/s2s/api/v1/shares
```

POST to notify that the share was accepted
```
http://localhost/oc/ocs/v1.php/apps/files_sharing/s2s/api/v1/shares/{shareId}/accept
```

POST to notify that the share was declined
```
http://localhost/oc/ocs/v1.php/apps/files_sharing/s2s/api/v1/shares/{shareId}/decline
```