Add additional constants to OAuth2ParameterNames

These additional constants are used for optional parameters in the
Access Token Request for the OAuth 2.0 Token Exchange Grant.

Issue gh-5199

Author: sjohnr

oauth2/oauth2-core/src/main/java/org/springframework/security/oauth2/core/endpoint/OAuth2ParameterNames.java

@@ -182,6 +182,18 @@ public final class OAuth2ParameterNames {
 	 */
 	public static final String INTERVAL = "interval";
 
+	/**
+	 * {@code audience} - used in Token Exchange Access Token Request.
+	 * @since 6.3
+	 */
+	public static final String AUDIENCE = "audience";
+
+	/**
+	 * {@code resource} - used in Token Exchange Access Token Request.
+	 * @since 6.3
+	 */
+	public static final String RESOURCE = "resource";
+
 	/**
 	 * {@code requested_token_type} - used in Token Exchange Access Token Request.
 	 * @since 6.3