feat: add annotations expression object for thymeleaf

[guqing]

What type of PR is this?

/kind feature
/area core
/milestone 2.1.x

What this PR does / why we need it:

新增操作 annotations 的表达式对象
在 thymeleaf 模板中使用示例：

<p th:text="${#annotations.get(user, 'background')}"></p>
<p th:text="${#annotations.getOrDefault(user, 'background', 'default-value')}"></p>
<p th:text="${#annotations.contains(user, 'background')}"></p>

Which issue(s) this PR fixes:

Fixes #3073

Special notes for your reviewer:

/cc @halo-dev/sig-halo

Does this PR introduce a user-facing change?

新增 Annotations 表达式对象用于在 thymeleaf 中操作自定义模型的 annotations

[ruibaby]

/lgtm

非常有用！

<div
-  th:style="|background:${categoryItem.metadata.annotations != null ? (categoryItem.metadata.annotations['bgColor'] ?: '#fff') : ''}|"
+  th:style="|background:${#annotations.getOrDefault(categoryItem, 'bgColor', '#000')}|"
></div>

[JohnNiang]

/approve

[f2c-ci-robot]

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This pull-request has been approved by: JohnNiang

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