feature: add support to read certificate private keys from file-system

[hbagdi]

Certificates are sensitive and can't be stored in plain-text.
In most use-cases, decK runs in a CI pipeline. In such a system, certificate private key and certificate can be provided to decK during runtime by CI via a file.
This avoids storing the certificate's private key in plaint-text in decK's state file.

[hbagdi]

Awaiting real user need. If there are enough users who need this,I'll develop and push this out.

[leolb-aphp]

Well in Gitlab CI, that could be useful. But there's more secrets than just certificates, some plugins contain sensitive tokens as configuration.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[mflendrich]

Blocked on the decision regarding security implications: #394 (review)