Private components within public spaces

Is your feature request related to a problem? Please describe.
In some participatory contexts, it would be great if we had the possibility to create private components within the public participatory spaces.

Currently Decidim allows creating private spaces but it will hide everything from that space from the public eyes. In some contexts semi-transparent spaces would be more suitable.

Describe the solution you'd like
I would like to propose two features to accommodate this need:

1. Separate the private participant management from the participatory spaces to the participants section in the admin panel, here we could have e.g. "private groups" which would have a name and list of participants that belong to those groups
2. Add the possibility to limit component access to these groups, as we already have the feature to limit the participatory access for private participants

The first feature would obviously also need some changes in the private participants management that we have today in Decidim. Instead of defining the list of participants for each space, the admin user could select the one or more private participant groups that have access to the space.

The component access would be defined similarly but would require its own administration view.

Describe alternatives you've considered
Using authorizations but it becomes complex and limited pretty soon. May also cause additional administrative headache.

Additional context
N/A

Does this issue could impact on users private data?
Not really, other than managing the private participant groups which requires managing private data of users. But it is already possible to manage this data so there would be no additional impact to the current situation.

Funded by
Not funded right now, just writing this down to have it in the backlog