[Feature] S3 Storage API support

[ShellWen]

Describe the feature
Nowadays, this app supports multiple storage backends. I think it would be even better if it could add support for S3 and S3-compatible APIs. Many cloud service providers offer S3-compatible APIs, such as Tencent Cloud and Alibaba Cloud in China, as well as Cloudflare R2 and even AWS itself internationally. It would be great if we could back up data to an S3-compatible API backend.

[alexma233]

How's it going on ? @XayahSuSuSu

[543069760]

@alexma233 S3 has already been forked and implemented; it's currently a test version, which you can test. https://github.com/543069760/Android-DataBackup-S3