Pageant: Implement NamedPipes-based stream

[amtelekom]

Adds an implementation of the newer NamedPipes-based access to the Pageant Stream.

Right now, it can be switched to via feature, i.e. the binary crate imports

pageant = { version = "0.0.3", features = ["namedpipes"] }

to enable it.

While the original client has some backwards-compatibility for very old Windows versions (the comments in some of the called code speak of Win9x and XP SP2), I've just implemented the primary path, which should work on all platforms the current MSRV supports (7+).

This is a breaking change both on the API-Surface of the Pageant crate (new needs to be async + have error handling), as well as in russh (also to introduce a Result return type).

Alternatively to this implementation, we could also just offer the generation of the Named-Pipe path in this crate and let users create a NamedPipe-Backed agent using the existing code in Russh, although this version seemed more comfortable to me.

[Eugeny]

Thanks a lot!

The two implementations should definitely not be mutually exclusive. They could be both exported together, with neither being a "default", e.g.:

#[cfg(all(windows, feature = "wmmessage"))]
pub use pageant_impl::PageantStream as PageantWmMessageStream;

#[cfg(all(windows, feature = "namedpipes"))]
pub use pageant_impl_namedpipes::PageantStream as PageantNamedPipeStream;

I'd really like to see a common trait for both of them since it would be weird to have to choose a specific implementation at compile time unless you know exactly which PuTTY version to expect in advance

[Rondom]

There is a cost to maintaining both and why would one want to run an old pre-2020 Putty Pageant with potential security bugs in 2025? I think a compile time decision is good enough, and I would even evaluate to eventually remove the WM_COPYDATA method to simplify maintenance.

[Eugeny]

Because in reality, when you're writing an SSH client, you don't control what version of Putty the user might have installed.

[Eugeny]

P.S. I don't mind doing it myself, it will just take a bit until I have time to work on it.