feat: MySQL should check client certificate (#1384)

PostgreSQL checks client certificates. MySQL should do the same.
Has additional benefit that in the GCP console it is reported
that only TLS connections are accepted. (Currently only TLS connections
are accepted because this is added as a setting to every user, but as
it's not shown in the console, it can look like the database would
accept insecure connections)

terraform/cloudsql/mysql/provision/main.tf

@@ -17,6 +17,8 @@ resource "google_sql_database_instance" "instance" {
       ipv4_enabled    = var.public_ip
       private_network = local.authorized_network_id
 
+      ssl_mode = var.allow_insecure_connections ? "ALLOW_UNENCRYPTED_AND_ENCRYPTED" : "TRUSTED_CLIENT_CERTIFICATE_REQUIRED"
+
       dynamic "authorized_networks" {
         for_each = var.authorized_networks_cidrs
         iterator = networks