Add deprecation check for jwt_header setting (opensearch-project#3887)

### Description
Add deprecation check for `jwt_header` setting

### Issues Resolved
- Related opensearch-project#3886

### Check List
- [ ] ~New functionality includes testing~
- [ ] New functionality has been documented
- [X] Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and
signing off your commits, please check
[here](https://github.com/opensearch-project/OpenSearch/blob/main/CONTRIBUTING.md#developer-certificate-of-origin).

---------

Signed-off-by: Peter Nied <petern@amazon.com>
Signed-off-by: Prabhas Kurapati <prabhask@berkeley.edu>

src/main/java/com/amazon/dlic/auth/http/jwt/AbstractHTTPJwtAuthenticator.java

@@ -28,6 +28,7 @@
 
 import org.opensearch.OpenSearchSecurityException;
 import org.opensearch.SpecialPermission;
+import org.opensearch.common.logging.DeprecationLogger;
 import org.opensearch.common.settings.Settings;
 import org.opensearch.common.util.concurrent.ThreadContext;
 import org.opensearch.core.common.Strings;
@@ -48,6 +49,7 @@
 
 public abstract class AbstractHTTPJwtAuthenticator implements HTTPAuthenticator {
     private final static Logger log = LogManager.getLogger(AbstractHTTPJwtAuthenticator.class);
+    private final static DeprecationLogger deprecationLog = DeprecationLogger.getLogger(AbstractHTTPJwtAuthenticator.class);
 
     private static final String BEARER = "bearer ";
     private static final Pattern BASIC = Pattern.compile("^\\s*Basic\\s.*", Pattern.CASE_INSENSITIVE);
@@ -75,6 +77,13 @@ public AbstractHTTPJwtAuthenticator(Settings settings, Path configPath) {
         requiredAudience = settings.get("required_audience");
         requiredIssuer = settings.get("required_issuer");
 
+        if (!jwtHeaderName.equals(AUTHORIZATION)) {
+            deprecationLog.deprecate(
+                "jwt_header",
+                "The 'jwt_header' setting will be removed in the next major version of OpenSearch.  Consult https://github.com/opensearch-project/security/issues/3886 for more details."
+            );
+        }
+
         try {
             this.keyProvider = this.initKeyProvider(settings, configPath);
             jwtVerifier = new JwtVerifier(keyProvider, clockSkewToleranceSeconds, requiredIssuer, requiredAudience);

src/main/java/com/amazon/dlic/auth/http/jwt/HTTPJwtAuthenticator.java

@@ -26,6 +26,7 @@
 
 import org.opensearch.OpenSearchSecurityException;
 import org.opensearch.SpecialPermission;
+import org.opensearch.common.logging.DeprecationLogger;
 import org.opensearch.common.settings.Settings;
 import org.opensearch.common.util.concurrent.ThreadContext;
 import org.opensearch.security.auth.HTTPAuthenticator;
@@ -44,6 +45,7 @@
 public class HTTPJwtAuthenticator implements HTTPAuthenticator {
 
     protected final Logger log = LogManager.getLogger(this.getClass());
+    protected final DeprecationLogger deprecationLog = DeprecationLogger.getLogger(this.getClass());
 
     private static final Pattern BASIC = Pattern.compile("^\\s*Basic\\s.*", Pattern.CASE_INSENSITIVE);
     private static final String BEARER = "bearer ";
@@ -69,6 +71,13 @@ public HTTPJwtAuthenticator(final Settings settings, final Path configPath) {
         requireAudience = settings.get("required_audience");
         requireIssuer = settings.get("required_issuer");
 
+        if (!jwtHeaderName.equals(AUTHORIZATION)) {
+            deprecationLog.deprecate(
+                "jwt_header",
+                "The 'jwt_header' setting will be removed in the next major version of OpenSearch.  Consult https://github.com/opensearch-project/security/issues/3886 for more details."
+            );
+        }
+
         final JwtParserBuilder jwtParserBuilder = KeyUtils.createJwtParserBuilderFromSigningKey(signingKey, log);
         if (jwtParserBuilder == null) {
             jwtParser = null;