feat: support JSON-based retrieval and update for theme and plugin configs

[guqing]

What type of PR is this?

/kind improvement /area core /milestone 2.20.x

What this PR does / why we need it:

新增 API 用于以 JSON 的方式获取和更新插件和主题的配置

之前一直是返回的 ConfigMap 让前端去操作字符串 JSON 需要转换，渲染表单值回显等操作都不方便，例如像 SEO 插件这样还会去获取配置在业务中使用而不仅仅是只有后端会使用到配置，因此使用 JSON 的方式返回可能要更合理一些

Does this PR introduce a user-facing change?

新增 API 用于以 JSON 格式获取和更新插件和主题的配置

[f2c-ci-robot[bot]]

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[guqing]

@ruibaby 帮忙适配一下前端部分

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 79.20000% with 26 lines in your changes missing coverage. Please review.

Project coverage is 58.26%. Comparing base (6cd8dc8) to head (a634065). Report is 72 commits behind head on main.

Files with missing lines	Patch %	Lines
...tension/service/impl/SettingConfigServiceImpl.java	5.26%	18 Missing :warning:
...un/halo/app/core/extension/theme/SettingUtils.java	0.00%	4 Missing :warning:
...lo/app/core/extension/endpoint/PluginEndpoint.java	96.07%	1 Missing and 1 partial :warning:
...n/halo/app/core/extension/theme/ThemeEndpoint.java	96.07%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6661 +/- ## ============================================ + Coverage 58.18% 58.26% +0.07% - Complexity 3774 3948 +174 ============================================ Files 651 675 +24 Lines 22125 23189 +1064 Branches 1538 1576 +38 ============================================ + Hits 12873 13510 +637 - Misses 8641 9059 +418 - Partials 611 620 +9 ```

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[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud