introduce no-tty feature

[handewo]

This PR adds an experimental no-tty Cargo feature that allows the crate to be used in some environments where spawning a new pseudo-terminal (PTY) per session is impossible—e.g. when using Tokio, which does not support fork.

Background & Motivation

• The russh ecosystem is async-only today.
• Tokio’s runtime is not fork-safe, so we cannot allocate a new PTY for every incoming SSH session inside the async worker.
• Without this feature, downstream users who only need exec/subsystem channels (and never interactive shells) are blocked from adopting the library.

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Key Changes

• Added optional feature no-tty in Cargo.toml.
• Gate all PTY allocation logic behind #[cfg(not(feature = "no-tty"))].
• Provide stub implementations that return ChannelFailure when a client requests a PTY while the feature is active.
• Added examples/ssh-service.rs that starts a minimal async SSH server with no-tty enabled for quick testing.

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Testing

1. Check out this branch
2. cargo run --example ssh-service --features no-tty
3. Connect with any SSH client:

   ssh 127.0.0.1 -p 2222 -o "UserKnownHostsFile=/dev/null" -o "StrictHostKeyChecking=no"

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