protected ExternalId externalId(VaultName vaultName) {
        return Optional.ofNullable(vaults.get(vaultName)).map(Vault::externalId).orElse(null);
    }
Secret reader for tenant nodes.

@author gjoranv
jdisc-cloud-aws/src/main/java/ai/vespa/secret/aws/AsmTenantSecretReader.java