Support encrypted client.key for remotes with keepalive configuration

[usrflo]

Is there an existing issue for this?

• There is no existing issue for this feature

What are you currently unable to do

Using an encrypted client.key for a remote that was configured with a keepalive interval via
incus remote add mysrv https://1.2.3.4:8443 --keepalive=30
causes a remote proxy to be started in the background when using an incus command like incus ls on this remote. The command doesn't terminate because the proxy process in the background waits for the key password input.

What do you think would need to be added

A change would be required that allows to enter the password before sending the proxy process to the background.