feat: add plugin finder for theme-side (#2683)

#### What type of PR is this?
/kind feature
/milestone 2.0
/area core
#### What this PR does / why we need it:
主题端提供判断插件是否已经启用的方法

#### Which issue(s) this PR fixes:
Fixes #2677

#### Special notes for your reviewer:
how to test it?
在主题端使用 `${pluginFinder.available(your-plugin-name)}`  来查看某个插件是否已经启用，如果是则得到 true ，否则 false，如果插件没有安装也得到 false

/cc @halo-dev/sig-halo 
#### Does this PR introduce a user-facing change?

```release-note
主题端支持获取插件启用状态
```

src/main/java/run/halo/app/theme/finders/PluginFinder.java

@@ -0,0 +1,12 @@
+package run.halo.app.theme.finders;
+
+/**
+ * A finder for {@link run.halo.app.core.extension.Plugin}.
+ *
+ * @author guqing
+ * @since 2.0.0
+ */
+public interface PluginFinder {
+
+    boolean available(String pluginName);
+}

src/main/java/run/halo/app/theme/finders/impl/PluginFinderImpl.java

@@ -0,0 +1,33 @@
+package run.halo.app.theme.finders.impl;
+
+import lombok.AllArgsConstructor;
+import org.apache.commons.lang3.StringUtils;
+import org.pf4j.PluginState;
+import org.pf4j.PluginWrapper;
+import run.halo.app.plugin.HaloPluginManager;
+import run.halo.app.theme.finders.Finder;
+import run.halo.app.theme.finders.PluginFinder;
+
+/**
+ * Plugin finder implementation.
+ *
+ * @author guqing
+ * @since 2.0.0
+ */
+@Finder("pluginFinder")
+@AllArgsConstructor
+public class PluginFinderImpl implements PluginFinder {
+    private final HaloPluginManager haloPluginManager;
+
+    @Override
+    public boolean available(String pluginName) {
+        if (StringUtils.isBlank(pluginName)) {
+            return false;
+        }
+        PluginWrapper pluginWrapper = haloPluginManager.getPlugin(pluginName);
+        if (pluginWrapper == null) {
+            return false;
+        }
+        return PluginState.STARTED.equals(pluginWrapper.getPluginState());
+    }
+}

src/test/java/run/halo/app/theme/finders/impl/PluginFinderImplTest.java

@@ -0,0 +1,50 @@
+package run.halo.app.theme.finders.impl;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.when;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.Mockito;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.pf4j.PluginState;
+import org.pf4j.PluginWrapper;
+import run.halo.app.plugin.HaloPluginManager;
+
+/**
+ * Tests for {@link PluginFinderImpl}.
+ *
+ * @author guqing
+ * @since 2.0.0
+ */
+@ExtendWith(MockitoExtension.class)
+class PluginFinderImplTest {
+    @Mock
+    private HaloPluginManager haloPluginManager;
+
+    @InjectMocks
+    private PluginFinderImpl pluginFinder;
+
+    @Test
+    void available() {
+        assertThat(pluginFinder.available(null)).isFalse();
+
+        boolean available = pluginFinder.available("fake-plugin");
+        assertThat(available).isFalse();
+
+        PluginWrapper mockPluginWrapper = Mockito.mock(PluginWrapper.class);
+        when(haloPluginManager.getPlugin(eq("fake-plugin")))
+            .thenReturn(mockPluginWrapper);
+
+        when(mockPluginWrapper.getPluginState()).thenReturn(PluginState.RESOLVED);
+        available = pluginFinder.available("fake-plugin");
+        assertThat(available).isFalse();
+
+        when(mockPluginWrapper.getPluginState()).thenReturn(PluginState.STARTED);
+        available = pluginFinder.available("fake-plugin");
+        assertThat(available).isTrue();
+    }
+}