feat: enable certificates relation on istio-pilot

Enabling the certificates relation enables the integration with tls-certificates-operator
and other charms that provide CA certificates. These CA certificates are used by istio-pilot
to configure TLS on the ingress Gateway resource.
In the past this was done exclusively via charm configuration, which is now removed in favour
of CA certificates provided through the certificates relation.

charms/istio-pilot/config.yaml

@@ -19,17 +19,3 @@ options:
     type: string
     default: istio-ingressgateway-workload
     description: Name of the service created by istio-gateway to use as a Gateway
-  ssl-crt:
-    type: string
-    default: ''
-    description: |
-        Base-64 certificate output. Can be set as follows:
-          $ juju config ssl-crt="$(cat CERT_FILE | base64 -w0)"
-        or on the bundle with: "include-base64://"
-  ssl-key:
-    type: string
-    default: ''
-    description: |
-        Base-64 key output. Can be set as follows:
-          $ juju config ssl-key="$(cat CERT_FILE | base64 -w0)"
-        or on the bundle with: "include-base64://"