Support retrieval of signing key in `apt.repo` operation (replace `apt.key`)

Describe the bug

The apt.key operation invokes the Debian apt-key command[1], but that command is deprecated (see e.g. the apt-key man page).

The man page above says apt-key will be removed from versions of Ubuntu >= 22.04. This isn't actually true – as I can confirm apt-key in fact is available on Ubuntu 22.04 – but it might be helpful to users to add a deprecation warning in the docstring for the operation, similar to what the equivalent Ansible command does:

https://github.com/ansible/ansible/blob/c3926268e226da771943cbdb96f65fd343a980d5/lib/ansible/modules/apt_key.py#L30

[1]: here, and on the following lines:

https://github.com/Fizzadar/pyinfra/blob/6eca1a52d955a0497cd33c02cb9a94176f93583d/pyinfra/operations/apt.py#L78

To Reproduce

N.A.

Expected behavior

It would be helpful to users to add a deprecation warning in the documentation.

Comments

The recommended new approach is to download keys to somewhere like /usr/share/keyrings, and to use the signed-by option in /etc/apt/sources files (see e.g. "apt-key is deprecated", based on information from the Debian wiki). So it might be worth adding a link to pyinfra's GPG operation documentation.

Inspect the apt.key operation docstring in pyinfra/operations/apt.py around the referenced lines. Compare the deprecation wording with Ansible's apt_key documentation and the linked GPG operation documentation; done means users are warned about apt-key and directed to the recommended keyring and signed-by approach.