feat: add an API for moment detail (#80)

#### What type of PR is this?

/kind feature

#### What this PR does / why we need it:

提供获取单条瞬间详细信息的接口，此接口返回数据包括瞬间作者信息及点赞数、评论数等。

#### How to test it?

测试接口 `/apis/console.api.moment.halo.run/v1alpha1/moments/{name}` 是否可用。

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

Fixes #74 

#### Does this PR introduce a user-facing change?
```release-note
提供获取单条瞬间详细信息的接口
```

src/main/java/run/halo/moments/ListedMoment.java

@@ -14,12 +14,12 @@
 @Builder
 public class ListedMoment {
 
-    @Schema(required = true)
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
     private Moment moment;
 
-    @Schema(required = true)
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
     private Contributor owner;
 
-    @Schema(required = true)
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
     private Stats stats;
 }

src/main/java/run/halo/moments/MomentEndpoint.java

@@ -47,6 +47,20 @@ public RouterFunction<ServerResponse> endpoint() {
                     );
                 QueryParamBuildUtil.buildParametersFromType(builder, MomentQuery.class);
             })
+            .GET("moments/{name}", this::getMoment,
+                builder -> builder.operationId("GetMoment")
+                    .description("Get a moment by name.")
+                    .tag(tag)
+                    .parameter(parameterBuilder()
+                        .name("name")
+                        .in(ParameterIn.PATH)
+                        .description("Moment name")
+                        .required(true)
+                        .implementation(String.class)
+                    )
+                    .response(responseBuilder()
+                        .implementation(ListedMoment.class)
+                    ))
             .GET("tags", this::listTags,
                 builder -> builder.operationId("ListTags")
                     .description("List all moment tags.")
@@ -78,6 +92,12 @@ public RouterFunction<ServerResponse> endpoint() {
             .build();
     }
 
+    private Mono<ServerResponse> getMoment(ServerRequest request) {
+        var name = request.pathVariable("name");
+        return momentService.findMomentByName(name)
+            .flatMap(moment -> ServerResponse.ok().bodyValue(moment));
+    }
+
     @Override
     public GroupVersion groupVersion() {
         return GroupVersion.parseAPIVersion("console.api.moment.halo.run/v1alpha1");

src/main/java/run/halo/moments/exception/NotFoundException.java

@@ -0,0 +1,15 @@
+package run.halo.moments.exception;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.web.server.ResponseStatusException;
+
+/**
+ * NotFoundException.
+ *
+ * @author LIlGG
+ */
+public class NotFoundException extends ResponseStatusException {
+    public NotFoundException(String reason) {
+        super(HttpStatus.NOT_FOUND, reason);
+    }
+}

src/main/java/run/halo/moments/service/MomentService.java

@@ -19,4 +19,6 @@ public interface MomentService {
     Mono<Moment> create(Moment moment);
 
     Flux<String> listAllTags();
+
+    Mono<ListedMoment> findMomentByName(String name);
 }

src/main/java/run/halo/moments/service/impl/MomentServiceImpl.java

@@ -8,6 +8,7 @@
 import org.springframework.security.core.context.ReactiveSecurityContextHolder;
 import org.springframework.stereotype.Component;
 import org.springframework.util.Assert;
+import org.springframework.web.server.ServerWebInputException;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 import run.halo.app.core.extension.Counter;
@@ -20,6 +21,7 @@
 import run.halo.moments.Moment;
 import run.halo.moments.MomentQuery;
 import run.halo.moments.Stats;
+import run.halo.moments.exception.NotFoundException;
 import run.halo.moments.service.MomentService;
 import run.halo.moments.util.MeterUtils;
 
@@ -76,6 +78,13 @@ public Flux<String> listAllTags() {
             .distinct();
     }
 
+    @Override
+    public Mono<ListedMoment> findMomentByName(String name) {
+        return client.fetch(Moment.class, name)
+            .switchIfEmpty(Mono.error(new NotFoundException("Moment not found.")))
+            .flatMap(this::toListedMoment);
+    }
+
     private Mono<ListedMoment> toListedMoment(Moment moment) {
         ListedMoment.ListedMomentBuilder momentBuilder = ListedMoment.builder()
             .moment(moment);