The FTPS and WebDAV authentication handlers compared the client-supplied secret
key against the stored secret with `String::eq`, which short-circuits on the
first differing byte. A network attacker who knows (or enumerates) a valid
access key can recover the secret key byte-by-byte via response-timing analysis;
neither path is rate limited.
Switch both to a constant-time comparison using `subtle::ConstantTimeEq`, the
same primitive the SFTP handler and `rustfs/src/auth.rs::constant_time_eq`
already use. `subtle` is added to the `ftps` and `webdav` feature dependency
sets (it was previously gated on `sftp` only).
Addresses GHSA-3p3x-734c-h5vx.
