Users can easily create a certificate chain for testing using SignedCertificateExtension (#6519)

Motivation:

This is the first of a series of PRs included by
#6516.

Currently, our mTLS tests are not realistic in that:
- Certificate chains aren't verified
- We set TLS configurations to not verify the peer, which means in
reality there's no point in using mTLS.

For this, I propose that a `SignedCertificate` is introduced. This is a
certificate which is signed by another certificate (another
`SignedCertificate` or `SelfSignedCertificate`). A JUnit `Extension` has
also been introduced so users can easily test this behavior.

In the process, I found that `SelfSingedCertificateNameType` has a typo,
and isn't adding much value. This enum has been removed.

Modifications:

- Introduced `SignedCertificate`, `SignedCertificateExtension`
- `SelfSignedCertificate`, `SelfSignedCertificateExtension` inherits the
newly introduced constructs
- Default behavior has been modified so that all generated certificates
can act as a ca.
- `SelfSingedCertificateNameType` has been removed

Result:

- Users can easily create a certificate chain for testing using
`SignedCertificateExtension`

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Author: jrhee17

core/src/main/java/com/linecorp/armeria/internal/common/util/CertificateParams.java

@@ -0,0 +1,116 @@
+/*
+ * Copyright 2025 LY Corporation
+ *
+ * LY Corporation licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+
+package com.linecorp.armeria.internal.common.util;
+
+import java.security.PrivateKey;
+import java.security.cert.CertificateException;
+import java.security.cert.X509Certificate;
+import java.util.Date;
+import java.util.Random;
+
+import org.bouncycastle.asn1.x500.X500Name;
+import org.bouncycastle.cert.jcajce.JcaX509CertificateHolder;
+
+import com.linecorp.armeria.common.annotation.Nullable;
+
+final class CertificateParams {
+
+    private final String fqdn;
+    private final X500Name ownerName;
+    private final Random random;
+    private final int bits;
+    private final Date notBefore;
+    private final Date notAfter;
+    private final String algorithm;
+
+    @Nullable
+    private final X509Certificate issuerCert;
+    @Nullable
+    private final PrivateKey issuerPrivateKey;
+    private final X500Name issuerName;
+
+    CertificateParams(String fqdn, Random random, int bits, Date notBefore, Date notAfter,
+                      String algorithm) {
+        this.fqdn = fqdn;
+        this.random = random;
+        this.bits = bits;
+        this.notBefore = notBefore;
+        this.notAfter = notAfter;
+        this.algorithm = algorithm;
+        issuerCert = null;
+        issuerPrivateKey = null;
+
+        ownerName = new X500Name("CN=" + fqdn);
+        issuerName = ownerName;
+    }
+
+    CertificateParams(String fqdn, Random random, int bits, Date notBefore, Date notAfter,
+                      SignedCertificate issuer)
+            throws CertificateException {
+        this.fqdn = fqdn;
+        ownerName = new X500Name("CN=" + fqdn);
+
+        this.random = random;
+        this.bits = bits;
+        this.notBefore = notBefore;
+        this.notAfter = notAfter;
+        algorithm = issuer.key().getAlgorithm();
+        issuerCert = issuer.cert();
+        issuerPrivateKey = issuer.key();
+        issuerName = new JcaX509CertificateHolder(issuerCert).getSubject();
+    }
+
+    String fqdn() {
+        return fqdn;
+    }
+
+    Random random() {
+        return random;
+    }
+
+    int bits() {
+        return bits;
+    }
+
+    Date notBefore() {
+        return notBefore;
+    }
+
+    Date notAfter() {
+        return notAfter;
+    }
+
+    String algorithm() {
+        return algorithm;
+    }
+
+    @Nullable PrivateKey issuerPrivateKey() {
+        return issuerPrivateKey;
+    }
+
+    @Nullable X509Certificate issuerCert() {
+        return issuerCert;
+    }
+
+    X500Name issuerName() {
+        return issuerName;
+    }
+
+    X500Name ownerName() {
+        return ownerName;
+    }
+}