implement simplified PluginManager API

scm-webapp/src/main/java/sonia/scm/plugin/PluginCenterLoader.java

@@ -0,0 +1,42 @@
+package sonia.scm.plugin;
+
+import com.google.common.annotations.VisibleForTesting;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import sonia.scm.net.ahc.AdvancedHttpClient;
+
+import javax.inject.Inject;
+import java.io.IOException;
+import java.util.Collections;
+import java.util.Set;
+
+class PluginCenterLoader {
+
+  private static final Logger LOG = LoggerFactory.getLogger(PluginCenterLoader.class);
+
+  private final AdvancedHttpClient client;
+  private final PluginCenterDtoMapper mapper;
+
+  @Inject
+  public PluginCenterLoader(AdvancedHttpClient client) {
+    this(client, PluginCenterDtoMapper.INSTANCE);
+  }
+
+  @VisibleForTesting
+  PluginCenterLoader(AdvancedHttpClient client, PluginCenterDtoMapper mapper) {
+    this.client = client;
+    this.mapper = mapper;
+  }
+
+  Set<AvailablePlugin> load(String url) {
+    try {
+      LOG.info("fetch plugins from {}", url);
+      PluginCenterDto pluginCenterDto = client.get(url).request().contentFromJson(PluginCenterDto.class);
+      return mapper.map(pluginCenterDto);
+    } catch (IOException ex) {
+      LOG.error("failed to load plugins from plugin center, returning empty list");
+      return Collections.emptySet();
+    }
+  }
+
+}