feat(sftp): add macOS and Windows platform support (#3372)

The session watchdog now selects its detection method per
platform. It previously probed kernel TCP state through a
Linux-only procfs path, so on macOS every healthy idle session
was killed within a minute, and on Windows the watchdog never
spawned at all, leaving wedged sessions with no cleanup. Linux
keeps its fast-kill watchdog unchanged. Other platforms get a
silence-only backstop that kills a session only at the
documented 30-minute idle ceiling.

The host-key loader now has a Windows arm. It loads OpenSSH
format host keys from the configured directory and logs a
one-time warning to restrict NTFS ACLs on the key directory,
the same operator-managed approach FTPS, WebDAV, KMS, and IAM
already use on Windows. Startup previously aborted with
UnsupportedPlatform because the Unix mode-bit permission check
has no Windows equivalent. tokio's io-uring feature is now
enabled only in Linux builds. io-uring is a Linux kernel
interface and enabling it unconditionally broke the Windows
build.

Co-authored-by: houseme <housemecn@gmail.com>

rustfs/Cargo.toml

@@ -112,7 +112,7 @@ http-body.workspace = true
 http-body-util.workspace = true
 reqwest = { workspace = true }
 socket2 = { workspace = true }
-tokio = { workspace = true, features = ["rt-multi-thread", "macros", "net", "signal", "process", "io-util", "io-uring"] }
+tokio = { workspace = true, features = ["rt-multi-thread", "macros", "net", "signal", "process", "io-util"] }
 tokio-rustls = { workspace = true }
 aws-sdk-s3 = { workspace = true }
 tokio-stream.workspace = true
@@ -177,6 +177,8 @@ hashbrown = { workspace = true }
 
 [target.'cfg(target_os = "linux")'.dependencies]
 libsystemd.workspace = true
+# io-uring is Linux-only. Scope the feature to Linux targets.
+tokio = { workspace = true, features = ["io-uring"] }
 
 [target.'cfg(not(all(target_os = "linux", target_env = "gnu", target_arch = "x86_64")))'.dependencies]
 mimalloc = { workspace = true }