Host keys are not checked when using SFTP

[happyc0ding]

https://github.com/juicedata/juicefs/blob/53f57ac4b06ce52d069b396c89d3e9e1487e3b7c/pkg/object/sftp.go#L532C20-L532C47 uses ssh.InsecureIgnoreHostKey().

As https://pkg.go.dev/golang.org/x/crypto/ssh#InsecureIgnoreHostKey states:

InsecureIgnoreHostKey returns a function that can be used for ClientConfig.HostKeyCallback to accept any host key. It should not be used for production code.

From a security perspective it is highly recommended to use the common methods (i.e. ~/.ssh/known_hosts) to read and verify known hosts or at least to provide a parameter/environment variable to provide a path.
This is (almost) worth a CVE these days since this is quite the same as ignoring certificates when using HTTPS.

[zhijian-pro]

We will add a parameter to support this verification