Add support for cert-authority

[bwitt]

Add support for ssh_key type cert-authority by using a new marker param

Add tests and acceptance test too.

Example:

sshkey { '*.example.com':
  ensure => present,
  type   => 'ssh-rsa',
  marker => 'cert-authority',   # or 'revoked'
  key    => 'AAAA…',
}

results in

@cert-authority *.example.com ssh-rsa <key>

Fixes #89

[bwitt]

@jst-cyr would it be possible to get this one reviewed? thanks!

[jst-cyr]

Running CI tests for this PR here: #117

[jst-cyr]

@deric : This one requires somebody from our Phoenix engineering team to do the code review.

I did kick off the acceptance testing and the results are all green.

Acceptance tests: https://github.com/puppetlabs/puppetlabs-sshkeys_core/actions/runs/32731008083