m/n/core/rpc: limit API footgun availability

This unifies the interface of the
New{Ephemeral,Authenticated}Credentials calls. They now use the same set
of CredentialsOpt options which allows both calls to request a
particular verification of the remote side of the connection.
NewEphemeralCredentials also now requires an explicit WantInsecure
option which surfaces attempts to dial the cluster without CA/node
verification.

Change-Id: Ibb65cb0952f6ff2092a3f55fe1c5a31bd2b72b36
Reviewed-on: https://review.monogon.dev/c/monogon/+/2741
Tested-by: Jenkins CI
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/metropolis/cli/metroctl/core/rpc.go b/metropolis/cli/metroctl/core/rpc.go
index 4aeda75..e519906 100644
--- a/metropolis/cli/metroctl/core/rpc.go
+++ b/metropolis/cli/metroctl/core/rpc.go
@@ -48,7 +48,7 @@
 	}
 
 	if ocert == nil {
-		creds, err := rpc.NewEphemeralCredentials(opkey, nil)
+		creds, err := rpc.NewEphemeralCredentials(opkey, rpc.WantInsecure())
 		if err != nil {
 			return nil, fmt.Errorf("while building ephemeral credentials: %v", err)
 		}