feat(services/sftp): support copy and read_seek

[silver-ymz]

Update:

• add copyable setting in SftpBuilder to make user provide info about remote server extension
• bump openssh-sftp-client version to 0.13.5 to fix the bug of AsyncSeek
• add read_can_seek in Capability

[suyanhanx]

Return an error when call copy if user did not enable copy?

[Xuanwo]

Return an error when call copy if user did not enable copy?

We will handle this in general in the future, for example, check capability before calling.

[silver-ymz]

Return an error when call copy if user did not enable copy?

https://github.com/apache/incubator-opendal/blob/9cce4d31418567ed2fc8e7178f11a48ac9cbdede/core/src/services/sftp/backend.rs#L391

https://github.com/apache/incubator-opendal/blob/9cce4d31418567ed2fc8e7178f11a48ac9cbdede/core/src/services/sftp/error.rs#L34

Current implementation is to return an error of Unsupported if the remote server doesn't support.

[Xuanwo]

Thanks!

[silver-ymz]

And may I ask is there anything else need to do about sftp? I think all available features have done.

[Xuanwo]

And may I ask is there anything else need to do about sftp?

I guess we don't need to do more until we have user's feature request.