We generate certificates that are intended to be used for server authentication in a couple of places:

- `bin/elasticsearch-certutil http` command  ( See `CertGenUtils.generateSignedCertificate` in `HttpCertificateCommand` )
- TLS auto-configuraion for node startup ( see [`AutoConfigureNode`](https://github.com/elastic/elasticsearch/blob/814f7f9f52cb8c05db6465b9bfc818ea5d76c575/x-pack/plugin/security/cli/src/main/java/org/elasticsearch/xpack/security/cli/AutoConfigureNode.java#L454) )

In these cases we know that the certificate is meant to be used for server authentication, so we can set Extended Key Usage extension to `1.3.6.1.5.5.7.3.1`
x-pack/plugin/security/cli/src/main/java/org/elasticsearch/xpack/security/cli/CertGenUtils.java
PrivateKey caPrivKey,
        boolean isCa,
        int days,
        String signatureAlgorithm
    ) throws NoSuchAlgorithmException, CertificateException, CertIOException, OperatorCreationException {
        Objects.requireNonNull(keyPair, "Key-Pair must not be null");
        final ZonedDateTime notBefore = ZonedDateTime.now(ZoneOffset.UTC);
        if (days < 1) {
            throw new IllegalArgumentException("the certificate must be valid for at least one day");
        }
        final ZonedDateTime notAfter = notBefore.plusDays(days);
        return generateSignedCertificate(
            principal,
            subjectAltNames,
            keyPair,
            caCert,
            caPrivKey,
            isCa,
            notBefore,
            notAfter,
            signatureAlgorithm
        );
    }
    public static X509Certificate generateSignedCertificate(
        X500Principal principal,
        GeneralNames subjectAltNames,
        KeyPair keyPair,
        X509Certificate caCert,
        PrivateKey caPrivKey,
        boolean isCa,