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/test/java/org/elasticsearch/xpack/security/cli/CertGenUtilsTests.java
null,
            rootCaKeyPair.getPrivate(),
            true,
            notBefore,
            notAfter,
            null
        );
        
        final X500Principal subCaPrincipal = new X500Principal("DC=Sub CA,DC=example.com");
        final KeyPair subCaKeyPair = CertGenUtils.generateKeyPair(2048);
        final X509Certificate subCaCert = CertGenUtils.generateSignedCertificate(
            subCaPrincipal,
            null,
            subCaKeyPair,
            rootCaCert,
            rootCaKeyPair.getPrivate(),
            true,
            notBefore,
            notAfter,
            null
        );
        
        final X500Principal endEntityPrincipal = new X500Principal("CN=TLS Client\\+Server,DC=Sub CA,DC=example.com");
        final KeyPair endEntityKeyPair = CertGenUtils.generateKeyPair(2048);
        final X509Certificate endEntityCert = CertGenUtils.generateSignedCertificate(
            endEntityPrincipal,
            null,
            endEntityKeyPair,
            subCaCert,
            subCaKeyPair.getPrivate(),