feat: Compatibility with vault-acme (fixes #900)

[xeroc]

First approach to making issuing of certificates compatible with the way vault-acme works. Essentially, just allow to also load certificates from cert instead of certificate.

Since i am a Go novice, please let me know if there is a cleaner approach to this.

Also, am I supposed to direct PRs to master?

[CLAassistant]

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