Add support for Windows

[pronebird]

• OpenSSH Authentication Agent is disabled by default on Windows 11. Needs to be started manually
• The pipe path is \\.\pipe\openssh-ssh-agent but it's up to consumer to provide it anyway
• windows-sys is really only used because of one constant so I relaxed the requirement to >= 0.52 which is fairly old anyway but should provide a decent leeway for users of sshauth
• Tests pass!

$ ssh-keygen.exe -t ed25519 -C "PIV_slot_9A" -f C:\Users\hotshot\.ssh\piv_slot_9a_test_key
$ ssh-add.exe C:\Users\hotshot\.ssh\piv_slot_9a_test_key

$ cargo test
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.08s
     Running unittests src\lib.rs (target\debug\deps\sshauth-529ce9341cd934a3.exe)

running 7 tests
test agent::test::listing_keys ... ok
test token::test::encode ... ok
test token::test::old_encoding ... ok
test token::test::round_trip ... ok
test test::file_signing ... ok
test test::old_sigs ... ok
test agent::test::signing_something ... ok

test result: ok. 7 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.10s

Closes #9