Add ability to set attributes on a SslContext (#9654)

Motivation:

Sometimes it is useful to be able to set attributes on a SslContext.

Modifications:

Add new method that will return a AttributeMap that is tied to a SslContext instance

Result:

Fixes https://github.com/netty/netty/issues/6542.

handler/src/main/java/io/netty/handler/ssl/SslContext.java

@@ -24,6 +24,8 @@ import io.netty.channel.ChannelPipeline;
 import io.netty.handler.ssl.ApplicationProtocolConfig.Protocol;
 import io.netty.handler.ssl.ApplicationProtocolConfig.SelectedListenerFailureBehavior;
 import io.netty.handler.ssl.ApplicationProtocolConfig.SelectorFailureBehavior;
+import io.netty.util.AttributeMap;
+import io.netty.util.DefaultAttributeMap;
 import io.netty.util.internal.EmptyArrays;
 
 import java.security.Provider;
@@ -97,6 +99,7 @@ public abstract class SslContext {
     }
 
     private final boolean startTls;
+    private final AttributeMap attributes = new DefaultAttributeMap();
 
     /**
      * Returns the default server-side implementation provider currently in use.
@@ -862,6 +865,13 @@ public abstract class SslContext {
         this.startTls = startTls;
     }
 
+    /**
+     * Returns the {@link AttributeMap} that belongs to this {@link SslContext} .
+     */
+    public final AttributeMap attributes() {
+        return attributes;
+    }
+
     /**
      * Returns {@code true} if and only if this context is for server-side.
      */