Service API: Get usergroups that have projects (and sites) access

[SBriere]

Is your feature request related to a problem? Please describe.
A service might requires to get all usergroups that have access to a specific project / site in order to remove access if the project / site is deleted by the service.

Describe the solution you'd like
Expand the ServiceSiteProjectAccessRole API to include that feature or implement a new API.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.