`step_ca_certificate`: add `state` parameter

[maxhoesel]

Right now, the step_ca_certificate module simply calls step-cli without checking for any existing certificates.
This results in the following behavior:

• If force is false: On the second run, step-cli asks for confirmation for overwriting the existing cert, causing the module to fail.
• If force is true: The certificate is always overwritten, even when this is not needed.

Neither of these are really ideal from a user POV.
As a user, I would like the step_ca_certificate module to ensure that a valid certificate with the desired properties exists on the system, regardless of the underlying details.
Such a valid certificate must:

• Be present at the given path
• Have correct parameters, such as SANs
• Be valid (not expired)

It should be step_ca_certificates job to ensure that these conditions are met, whether that includes creating a new certificate or not.

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I propose the following approach to accomplish this:

• Introduce the serial_number parameter and other parameters from step_ca_revoke
• Introduce the revoke_on_delete parameter
• Introduce a state parameter with the following options:
  • present
    - If the certificate doesn't exist, is expired or has a SAN parameter mismatch, create a new certificate
    - Else, do nothing
  • revoked
    • Use either the path or the serial number and ensure that the given certificate is revoked.
  • absent
    • Delete the certificate from the system
      • If revoke_on_delete is true, revoke the certificate first
• Change the force parameter behavior: Instead of passing force on the command line to replace existing files (which we pretty much always want given the above model), use of force with present will now generate a new certificate on every single run.

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Implementing this feature would require a few steps:

• Implement checks for certificate validity:
  • Exists
  • Parameters correct
  • Valid
• Implement functionality for:
  • Revoke
  • Delete
  • Force behavior change
• Implementing tests to ensure correct behavior
• Remove the step_ca_certificate_revoke module