extension design and registration (#3)

#### What type of PR is this?
/kind feature
#### What this PR does / why we need it:
此 PR 完成了代码的自模型定义和注册
#### Which issue(s) this PR fixes:
Fixes #2 
#### Does this PR introduce a user-facing change?
```release-note
None
```

src/main/java/run/halo/injection/HtmlInjection.java

@@ -0,0 +1,48 @@
+package run.halo.injection;
+
+import static io.swagger.v3.oas.annotations.media.Schema.RequiredMode.REQUIRED;
+
+import io.swagger.v3.oas.annotations.media.Schema;
+import java.util.Set;
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+import run.halo.app.extension.AbstractExtension;
+import run.halo.app.extension.GVK;
+
+@Data
+@EqualsAndHashCode(callSuper = true)
+@GVK(group = "theme.halo.run",
+    version = "v1alpha1",
+    kind = "HtmlInjection",
+    plural = "htmlinjections",
+    singular = "htmlinjection")
+public class HtmlInjection extends AbstractExtension {
+
+    @Schema(requiredMode = REQUIRED)
+    private Spec spec;
+
+    @Data
+    public static class Spec {
+        @Schema(description = "The name of the code snippet", maxLength = 100)
+        private String name;
+
+        @Schema(description = "The description of the code snippet", maxLength = 500)
+        private String description;
+
+        @Schema(description = "The content of the HTML")
+        private String fragment;
+
+        @Schema(description = "where to inject", allowableValues = {"HEADER", "FOOTER"})
+        private InjectionPoint injectionPoint;
+
+        @Schema(description = "The pages where the code snippet should be injected")
+        private Set<String> pageRules;
+
+        @Schema(description = "Whether the code snippet is enabled", defaultValue = "false")
+        private boolean enabled;
+    }
+
+    public enum InjectionPoint {
+        HEADER, FOOTER
+    }
+}

src/main/java/run/halo/injection/InjectionPlugin.java

@@ -1,11 +1,29 @@
 package run.halo.injection;
-import org.pf4j.PluginWrapper;
+
 import org.springframework.stereotype.Component;
+import run.halo.app.extension.Scheme;
+import run.halo.app.extension.SchemeManager;
 import run.halo.app.plugin.BasePlugin;
+import run.halo.app.plugin.PluginContext;
+
+
 @Component
 public class InjectionPlugin extends BasePlugin {
-    public InjectionPlugin(PluginWrapper wrapper) {
-        super(wrapper);
+    private final SchemeManager schemeManager;
+
+    public InjectionPlugin(PluginContext pluginContext, SchemeManager schemeManager) {
+        super(pluginContext);
+        this.schemeManager = schemeManager;
+    }
+
+    @Override
+    public void start() {
+        schemeManager.register(HtmlInjection.class);
+    }
+
+    @Override
+    public void stop() {
+        schemeManager.unregister(Scheme.buildFromType(HtmlInjection.class));
     }
 }