chore: Replace abstract provider class with interface.

src/main/java/io/github/nstdio/http/ext/spi/CompositeJsonMappingProvider.java

@@ -19,7 +19,7 @@
 import static io.github.nstdio.http.ext.spi.Classpath.isGsonPresent;
 import static io.github.nstdio.http.ext.spi.Classpath.isJacksonPresent;
 
-class CompositeJsonMappingProvider extends JsonMappingProvider {
+class CompositeJsonMappingProvider implements JsonMappingProvider {
   private static final String NO_JSON_MAPPING_FOUND = "No JsonMapping implementation found. Please consider to add any of dependencies to classpath: 'com.fasterxml.jackson.core:jackson-databind', 'com.google.code.gson:gson'";
 
   static boolean hasAnyImplementation() {

src/main/java/io/github/nstdio/http/ext/spi/DefaultProviderHolder.java

@@ -0,0 +1,21 @@
+/*
+ * Copyright (C) 2022 Edgar Asatryan
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.github.nstdio.http.ext.spi;
+
+final class DefaultProviderHolder {
+  static final CompositeJsonMappingProvider DEFAULT_PROVIDER = new CompositeJsonMappingProvider();
+}

src/main/java/io/github/nstdio/http/ext/spi/JsonMappingProvider.java

@@ -23,21 +23,13 @@
 /**
  * The SPI to for {@linkplain JsonMapping}.
  */
-public abstract class JsonMappingProvider {
-
-  /**
-   * Inheritance constructor.
-   */
-  @SuppressWarnings("WeakerAccess")
-  protected JsonMappingProvider() {
-  }
-
+public interface JsonMappingProvider {
   /**
    * Finds the first available {@code JsonMappingProvider}.
    *
    * @return The {@code JsonMappingProvider}.
    */
-  public static JsonMappingProvider provider() {
+  static JsonMappingProvider provider() {
     return loader()
         .findFirst()
         .or(JsonMappingProvider::defaultProvider)
@@ -51,7 +43,7 @@ public static JsonMappingProvider provider() {
    *
    * @throws JsonMappingProviderNotFoundException When requested provider is not found.
    */
-  public static JsonMappingProvider provider(String name) {
+  static JsonMappingProvider provider(String name) {
     return loader()
         .stream()
         .filter(provider -> provider.type().getName().equals(name))
@@ -78,9 +70,5 @@ private static ServiceLoader<JsonMappingProvider> loader() {
    *
    * @return The {@code JsonMapping} instance.
    */
-  public abstract JsonMapping get();
-
-  private static final class DefaultProviderHolder {
-    private static final CompositeJsonMappingProvider DEFAULT_PROVIDER = new CompositeJsonMappingProvider();
-  }
+  JsonMapping get();
 }