In c244e5a6 this resource was converted to a data source, but that was
a mistake since data sources are expected to produce stable results on
each run, and yet certificate requests contain a random nonce as part of
the signature.
Additionally, using the data source as a managed resource through the
provided compatibility shim was not actually working, since "Read" was
trying to parse the private key out of a SHA1 hash of the key, which is
what we place in state due to the StateFunc on that attribute.
By restoring this we restore Terraform's ability to produce all of the
parts of a basic PKI/CA, which is useful for creating dev environments
and bootstrapping PKI for production environments.
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