pyrocufflink: Enable pam_ssh_agent_auth for sudo

By default, `sudo` requires users to authenticate with their passwords
before granting them elevated privileges.  It can be configured to
allow (some) users access to (some) privileged commands without
prompting for a password (i.e. `NOPASSWD`), however this has a real
security implication.  Disabling the password requirement would
effectively grant *any* program root privileges.  Prompting for a
password prevents malicious software from running privileged commands
without the user knowing.

Unfortunately, handling `sudo` authentication for Ansible is quite
cumbersome.  For interactive use, the `--ask-become-pass`/`-K` argument
is useful, though entering the password for each invocation of
`ansible-playbook` while iterating on configuration policy development
is a bit tedious.  For non-interactive use, though, the password of
course needs to be stored somewhere.  Encrypting it with Ansible Vault
is one way to protect it, but it still ends up stored on disk somewhere
and needs to be handled carefully.

*pam_ssh_agent_auth* provides an acceptable solution to both issues.  It
is better than disabling `sudo` authentication entirely, but a lot more
convenient than dealing with passwords.  It uses the calling user's SSH
agent to assert that the user has access to a private key corresponding
to one of the authorized public keys.  Using SSH agent forwarding, that
private key can even exist on a remote machine.  If the user does not
have a corresponding private key, `sudo` will fall back to normal
password-based authentication.

The security of this solution is highly dependent on the client to store
keys appropriately.  FIDO2 keys are supported, though when used with
Ansible, it is quite annoying to have to touch the token for _every
task_ on _every machine_.  Thus, I have created new FIDO2 keys for both
my laptop and my desktop that have the `no-touch-required` option
enabled.  This means that in order to use `sudo` remotely, I still need
to have my token plugged in to my computer, but I do not have to tap it
every time it's used.

For Jenkins, a hardware token is obviously impossible, but using a
dedicated key stored as a Jenkins credential is probably sufficient.

group_vars/pyrocufflink/main.yml

@@ -13,3 +13,12 @@ root_authorized_keys: |
   ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIJsL5fSylmiJmBtW0DH/viAAmtU2E/2M17GPvysiyRs+ dustin@rosalina
   ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIBw1T18jnBfR5reKAACOs/LMcs+jbclj6Eh8z56kJE7+ dustin@luma
   {% endif %}
+
+sudo_use_pam_ssh_agent: true
+sudo_authorized_ssh_keys: |
+  sk-ssh-ed25519@openssh.com AAAAGnNrLXNzaC1lZDI1NTE5QG9wZW5zc2guY29tAAAAIF4yQAS0bAQ9Ymxgxv828MsX0z4ff/Fs//0PQOtPexRJAAAABHNzaDo= dustin@rosalina.pyrocufflink.blue
+  sk-ssh-ed25519@openssh.com AAAAGnNrLXNzaC1lZDI1NTE5QG9wZW5zc2guY29tAAAAINal4+Gn/KuyP6YTsQuW4cphfDcjrS428osVIqnqMfagAAAABHNzaDo= dustin@luma.pyrocufflink.blue
+  ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIDD3Ebb7dyEyCylgEjmhFxvGqbPkT+0KSpI+xEGXLFnn jenkins
+# Default flags include -n, which makes Ansible complain about a "missing
+# become password," even though it would never actually prompt for one.
+ansible_become_flags: -H