Expose protected method in AbstractJackson2HttpMessageConverter

Closes gh-25509

spring-web/src/main/java/org/springframework/http/converter/json/AbstractJackson2HttpMessageConverter.java

@@ -285,7 +285,15 @@ public abstract class AbstractJackson2HttpMessageConverter extends AbstractGener
 		}
 	}
 
-	private static Charset getCharset(@Nullable MediaType contentType) {
+	/**
+	 * Return the charset to use for JSON input.
+	 * <p>By default this is either the charset from the input {@code MediaType}
+	 * or otherwise falling back on {@code UTF-8}.
+	 * @param contentType the content type of the HTTP input message
+	 * @return the charset to use
+	 * @since 5.1.18
+	 */
+	protected static Charset getCharset(@Nullable MediaType contentType) {
 		if (contentType != null && contentType.getCharset() != null) {
 			return contentType.getCharset();
 		}