step-ca: Deploy Step CA service

[Step CA] is an open-source online X.509 and SSH certificate authority
service.  It supports issuing certificates via various protocols,
including ACME and its own HTTP API via the `step` command-line utility.
Clients can authenticate using a variety of methods, such as JWK, Open
ID Connect, or mTLS.  This makes it very flexible and easy to introduce
to an existing ecosystem.

Although the CA service is mostly stateless, it does have an on-disk
database where stores some information, notably the list of SSH hosts
for which it has signed certificates.  Most other operations, though, do
not require any persistent state; the service does not keep track of
every single certificate it signed, for example.  It can be configured
to store authentication information (referred to as "provisioners") in
the database instead of the configuration file, by enabling the "remote
provisioner management" feature.  This has the advantage of being able
to modify authentication configuration without updating a Kubernetes
ConfigMap and restarting the service.

The official Step CA documentation recommends using the `step ca init`
command initialize a new certificate authority.  This command performs a
few steps:

* Generates an ECDSA key pair and uses it to create a self-signed root
  certificate
* Generates a second ECDSA key pair and signs an intermediate CA
  certificate using the root CA key
* Generates an ECDSA key pair and SSH root certificate
* Creates a `ca.json` configuration file

These steps can be performed separately, and in fact, I created the
intermediate CA certificate and signed it with the (offline) *dch Root
CA* certificate.

When the service starts for the first time, because
`authority/enableAdmin` is `true` and `authority/provisioners` is empty,
a new "Admin JWK" provisioner will be created automatically.  This key
will be encrypted with the same password used to encrypt the
intermediate CA certificate private key, and can be used to create other
provisioners.

[Step CA]: https://smallstep.com/docs/step-ca/

step-ca/intermediate_ca.crt

@@ -0,0 +1,13 @@
+-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----