feat: added sealed keyword

Author: opensourcevk

library/src/main/java/com/mastercard/developer/oauth2/core/access_token/AccessTokenStore.java

@@ -6,7 +6,7 @@
  * Interface for caching and retrieving OAuth 2.0 access tokens.
  * Implementations should handle token expiration and provide thread-safe operations.
  */
-public interface AccessTokenStore {
+public sealed interface AccessTokenStore  permits InMemoryAccessTokenStore {
     /**
      * Adds an access token to the store.
      */

library/src/main/java/com/mastercard/developer/oauth2/core/dpop/DPoPKeyProvider.java

@@ -3,7 +3,7 @@
 /**
  * DPoP key provider used to supply keys for creating DPoP proofs.
  */
-public interface DPoPKeyProvider {
+public sealed interface DPoPKeyProvider permits StaticDPoPKeyProvider {
     /**
      * Gets the current key to be used for signing DPoP proofs.
      */