Add Key module dependency to (optionally) store credentials outside the database

I have redone the patch for the latest version (8.x-1.2) as patch #24 wouldn't apply.
I have tweaked the code to make the use of the Key module optional.

I have been looking at how https://www.drupal.org/project/smtp/issues/3102055 ✨ Implement OAuth to support GMail/GSuite accounts Needs review have added support for a Gmail Oauth key authentication and think that utilising that code is a better way to go as it is also being actively developed. I have done a patch that is dependent on that code. If it gets merged in it makes it straight forward to add support for Key based authentication (and any other future authentication).

We've even running #26 for some time and found one small problem with it.

Here's the interdiff

65c65
< +    if ($smtp_key_id = $this->smtpConfig->get('smtp_key_id') && \Drupal::moduleHandler()->moduleExists('key')) {
---
> +    if (($smtp_key_id = $this->smtpConfig->get('smtp_key_id')) && \Drupal::moduleHandler()->moduleExists('key')) {