postgresql: Use a private CA-signed TLS cert

The PostgreSQL server managed by *Postgres Operator* uses a self-signed
certificate by default.  In order to enable full validation of the
server certificate, we need to use a certificate signed by a known CA
that the clients can trust.  To that end, I have added a *cert-manager*
Issuer specifically for PostgreSQL.  The CA certificate is also managed
by *cert-manager*; it is self-signed and needs to be distributed to
clients out-of-band.

postgresql/default-cluster.yaml

@@ -10,6 +10,8 @@ spec:
   numberOfInstances: 1
   postgresql:
     version: '15'
+  tls:
+    secretName: default-cert
   users:
     dustin:
     - superuser