Support getting parent category by child name in theme (halo-dev#5002)

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#### What this PR does / why we need it:
add getParentByName method to CategoryFinder interface

#### Which issue(s) this PR fixes:
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#### Special notes for your reviewer:
${categoryFinder.getParentByName(anyChild.metadata.name)}
#### Does this PR introduce a user-facing change?

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application/src/main/java/run/halo/app/theme/finders/CategoryFinder.java

@@ -28,4 +28,6 @@ public interface CategoryFinder {
     Flux<CategoryTreeVo> listAsTree();
 
     Flux<CategoryTreeVo> listAsTree(String name);
+
+    Mono<CategoryVo> getParentByName(String name);
 }

application/src/main/java/run/halo/app/theme/finders/impl/CategoryFinderImpl.java

@@ -81,6 +81,23 @@ public Flux<CategoryTreeVo> listAsTree(String name) {
         return this.toCategoryTreeVoFlux(name);
     }
 
+    @Override
+    public Mono<CategoryVo> getParentByName(String name) {
+        if (StringUtils.isBlank(name)) {
+            return Mono.empty();
+        }
+        return client.list(Category.class,
+                category -> {
+                    List<String> children = category.getSpec().getChildren();
+                    if (children == null) {
+                        return false;
+                    }
+                    return children.contains(name);
+                },
+                defaultComparator())
+            .next().map(CategoryVo::from);
+    }
+
     Flux<CategoryTreeVo> toCategoryTreeVoFlux(String name) {
         return listAll()
             .collectList()