Add build tag to omit the x/crypto/ssh dependency

[espadolini]

The only use of golang.org/x/crypto/ssh (and, transitively, of any package in crypto/...) is in the NewClient function, which is a small wrapper around running a subsystem request with an ssh.Client to then run a sftp client on the channel.

For binaries that don't otherwise depend on x/crypto/ssh (for example utility binaries that only do sftp through stdio) this can be a somewhat significant chunk of binary size; for example a stripped cgoless build of server_standalone (darwin arm64, go 1.26.1) goes from 2.9 to 2.0 MiB, which is (relatively) significant.

This PR moves NewClient to a new file which is omitted from the build if a new pkg_sftp.omit_ssh build tag is set during the build, so users of the library that don't use x/crypto/ssh can omit that dependency.