Activities. OCM. no event when a file / folder is shared with external users

## Describe the bug
There is no activity event triggered when a file is shared with an external user.

Taking into account that we're exposing information to the outside, it's important to know who shared the file, with who the file is shared, and what files are shared. This information should be easily accessible at a quick glance.
Right now, the information might be inferred (if only one space admin can share) or might be several clicks away, but the information is cumbersome to get.

## Steps to reproduce
1. Setup a federated connection through OCM between server1 and server2
2. User100 in server1 shares a file with user200 in server2
3.

## Expected behavior
A new activity event is shown for the file in server1. Anyone in server1 with access to that file can know that the user100 has shared the file with user200

## Actual behavior
There is no share event registered

## Setup
Mostly a default setup, with OCM and notifications enabled in oCIS, and activities in web

```console
OCIS_XXX=somevalue
OCIS_YYY=somevalue
PROXY_XXX=somevalue
```

## Additional context
The use case is the following:
1. We have admin1 and admin2, and several users in a particular space.
2. admin1 shares the file with edit permissions with a external user

There are a couple of problems with that:
* There is no indication to know whether the file is shared or not. The only way to know is to check the share menu, which is quite inconvenient:
* You might need to check the files one by one, with multiple clicks in between, to know which ones are shared with external users.
* The "shared with" might be long, and I'm not sure how easy would be to identify who is an external user and who not.
* There is no activity to know who shared the file. Even if only space admins are allowed to share within the space, if there are at least 2 space admins, there is no trace to know who shared the file and when the file was shared.