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'
Seems like I have problems to understand the use-case here, so from what I understand:

1. Bob creates a public share "foo" (with token "bar")
2. Bob sends a request to add the share to Alice
3. Alice clicks the "Accept share" button (which doesn't exist yet) and it gets added.

In step 3 to my understanding the token to accept the share is the one from point 1. Is that correct? If this is the case then Bob can also accept share's to Alice.
But it can also be that I just don't really get what this is used for :-)