Allow full configuration of provisioner webhooks in ca.json

[wouttom]

Hello!

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Issue details

We are currently running and configuring step-ca from infra-structure-as code. We want to add a webhook to a provisioner in the configurationfile ca.json.
Currently the entire webhook configuration is read from ca.json EXCEPT the secret that is used to sign the http-requests. The request will get signed by an empty secret if you configure the webhook in ca.json.

Make sure the secret is also read from ca.json.
To protect it and not store it plaintext/base64 encoded, it might be possible to encrypt the secret with the public key of the intermediate CA. Since the CA has the private key for its normal operations, it can decrypt the secret from the configfile.

Why is this needed?

To be to manage the configuration of the CA without having to run step cli-commands.
This allows for automating the configuration in a predictable way.

[areed]

Yes, we wanted to discourage keeping secrets in ca.json. Encrypting with the pub key of the intermediate is an interesting idea, but there may be risks that arise when using that keypair for more than just certificate signing. You can configure the webhook server to request and verify a client certificate if you want to be sure the request is coming from step-ca without using a secret.