m/proto: switch from CA pubkey to CA certificate in ClusterDirectory/Register

A CA certificate is a strict superset of the public key, and using it
instead of a public key allows us to connect to the cluster securely by
reusing standard/existing x509 CA auth, instead of having to implement a
check based on just a public key.

Backwards-incompatible proto change, but we're pre-MVP, and this flow is
just being implemented.

Change-Id: I014780a6ec3e5e8c6e81532531b18ad1438c8258
Reviewed-on: https://review.monogon.dev/c/monogon/+/424
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/metropolis/proto/api/management.proto b/metropolis/proto/api/management.proto
index 9f0e646..44db0c5 100644
--- a/metropolis/proto/api/management.proto
+++ b/metropolis/proto/api/management.proto
@@ -45,6 +45,6 @@
     // cluster that can be used to dial the cluster's services.
     metropolis.proto.common.ClusterDirectory cluster_directory = 1;
 
-    // ca_public_key is the Ed25519 public key of the CA of the cluster.
-    bytes ca_public_key = 2;
+    // ca_certificate is the x509 DER encoded CA certificate of the cluster.
+    bytes ca_certificate = 2;
 }