feat: Implement ETag support for app icon caching in GetAppIcon API

[HynoR]

What this PR does / why we need it?

Cache-Control 设置会让图片用缓存不变不更改策略
而Last-Modified设置的值又会在下次请求重新拉取
导致这两个字段冲突,只做到了看似有用的缓存效果。
复现方法：

1. 进入应用商店，访问第一页，然后切换到第二页。
2. 切换其他卡片，等待若干分钟
3. 再切回应用商店，第一页的图标仍然会请求而不是读缓存

Summary of your change

换成 etag 方案
一天内在缓存内读取，并保存 etag,一天过期后，请求后端会比对 etag,如果图片没变直接返回 304状态码，不返回内容让浏览器用缓存内容，减少网络传输的时间，同时保证版本更新。
效果:
接口返回的头：

过期后请求 ：

后端响应速度：

86400 一天缓存的值可以视情况改动，一般来说app 的 icon 很难变

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

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