[Feature] 支持51ape

**思路和建议**


众所周知的原因，因为没有vip，要么搜不到原唱，要么只能搜到128k 的MP3 。 其实大部分人只要320k 就可以，比如我。只能取51ape 下无损，假无损也不至于320k 都没有，所以应该还好

目的主要是为了替换网易云无版权歌曲。

**实现思路**

51ape 本身的搜索接口是坏的
- 只能通过google site 搜索，其实还是比较准的，基本都有
- 下载因为是度盘，只能通过 https://github.com/GangZhuo/BaiduPCS

The issue proposes adding a new source, 51ape, to the music-dl CLI tool. First, examine existing source modules (e.g., for NetEase, QQ) to understand the interface. The search requires using Google site search, and downloads involve BaiduPCS. Check if the project already handles Baidu downloads. 'Done' means a user can search and download from 51ape via the tool.

Written by the indexing model from the issue text.