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 give people a chance 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/cloud/shares
```

post data: shareWith, token, name, remote, remote_id, owner

POST to notify that the share was accepted
```
http://localhost/oc/ocs/v1.php/cloud/shares/{shareId}/accept
```

post data: token

POST to notify that the share was declined
```
http://localhost/oc/ocs/v1.php/cloud/shares/{shareId}/decline
```

post data: token

POST to unshare server-to-server share
```
http://localhost/oc/ocs/v1.php/cloud/shares/{shareId}/unshare
```
post data: 'token'
no, because the user doesn't have a account on the server. If I send you a remote share from my ownCloud and you accept it then there is nothing you can authenticate on my server because you don't have a account on it.

But the token is only known by you and me, and could be seen as some kind of "password". This should provide enough security to make sure that not a random person can accept/decline shares.