Considerations for opening up sharing across departments (self-serve option)

## Description

Clients can only self-serve add users from within their department. This was a decision made in Oct 2024 (when we shipped this feature), based on security/policy considerations:
> https://gcdigital.slack.com/archives/C01GFD150MC/p1780677781709429?thread_ts=1780677619.871449&cid=C01GFD150MC

## Impact

This is a restrictive measure that limits inter-departmental collaboration by design. Are there other security guardrails we can explore? How might we ensure that we’re supporting cross-collaboration across government? Especially to facilitate things for teams that often co-publish, for example: NSERC/SSHRC, HC/PHAC, etc.

## How might we prevent silos?
- Could we consider separating form collaboration and response data retrieval?
- Or differentiating permission access for key assets or for different data classifications?
- Might a first iteration focus on specific collaborations given patterns in which departments usually work together?

> https://gcdigital.slack.com/archives/C0117FN8G9K/p1780947063134429?thread_ts=1780946848.317849&cid=C0117FN8G9K