Improve cross-account and cross-domain consumer use

[ripienaar]

Proposed change

Today users have to assure stream+consumer name combinations are unique across accounts and domains if they wish to do imports between those.

Our FC and ACK subjects clash and do not have identifiers in them to make them unique.

We already have the ACK subject expended to include domains, the clients support it but the server does not have that feature yet. This would solve domains but not cross accounts but we might leverage that somehow to minimise the impact on the clients that would be very big.

See ADR-15 for currently support ACK subject designs

Use case

Coordinating cross acount/domain stream and consumer names should not be required.

Contribution

No response

[ripienaar]

Related #6431, #6440