[Feature]: Federation

### Problem description

Hello dear Defguard team,

would it be possible to implement a federation feature between instances, to use a gateway for Example to connect with multiple instances using one gateway?

We would like to connect to other companies with our own users, but with no need to deploy a dedicated gateway.

### Proposed solution

Federation feature between Instances to trust access from specific companies, who also run defguard.
Similar to nextcloud federation.
We need separate Access with own database for each company.

### Alternatives considered

Maybe there is an alternative already, which we don't know.
For Example deploying multiple Gateways on one host?
I Know this is possible, but we didn't manage to run multiple docker gateways on one machine yet.

### Impact

Important