Multiple attach should be supported

### Description

In terminal 1:

`nerdctl run --rm -ti --detach-keys=ctrl-a,ctrl-b --name what debian`

Do _not_ detach

Then in terminal 2:

`nerdctl attach what`

Start typing in terminal 2.

Notice that some characters are displayed on terminal 1, others on terminal 2.

Do the same with docker: it works.

There is something broken either in nerdctl, or maybe in `containerd/console`.

### Steps to reproduce the issue

_No response_

### Describe the results you received and expected

na

### What version of nerdctl are you using?

main

### Are you using a variant of nerdctl? (e.g., Rancher Desktop)

None

### Host information

_No response_