Is there an existing issue for this?

• There is no existing issue for this feature

What are you currently unable to do

Most folks will go through incus admin init on standalone systems to create and then join a cluster.
This works fine, except for environments where Incus is pre-initialized and is remote, like IncusOS.

For those, we should have a standalone incus cluster join which can be run following an incus cluster add on another system.

What do you think would need to be added

No response