Added a constant for DPOP in OAuth2AccessToken.TokenType

Issue gh-14915

Author: ThomasKasene

oauth2/oauth2-core/src/main/java/org/springframework/security/oauth2/core/OAuth2AccessToken.java

@@ -102,6 +102,7 @@ public static final class TokenType implements Serializable {
 		private static final long serialVersionUID = SpringSecurityCoreVersion.SERIAL_VERSION_UID;
 
 		public static final TokenType BEARER = new TokenType("Bearer");
+		public static final TokenType DPOP = new TokenType("DPoP");
 
 		private final String value;