RFC-2133: Introduce Append API

[ShadowySpirits]

Discussed in #2115

^{Originally posted by ShadowySpirits April 25, 2023}
OpenDAL provides append-alike operation, which is based on multipart API. But this approach has some limitations:

• Data could be lost and not readable before w.close() returned Ok(())
• File can't be append again after w.close() returned Ok(())

Currently, there are not many docs about this. To ensure proper usage, it is important to inform users about these risks. Furthermore, for services that support append API natively, using append API instead of multipart API is a better choice.

I propose to separate the write and append operations. The files created by the write operation can not be modified. And the files created by the append operation can be appended via multipart API or append API for services that natively support append, such as Azure blob and Alibaba cloud OSS.

[Xuanwo]

Thanks a lot!