Enable connections via a proxy

[timlyo]

This PR adds functionality for SSH'ing to a remote host via an SSH proxy.

I implemented this feature for my own project, happy to discuss changing any details if you're interested in merging it.

[Copilot]

Pull request overview

This PR implements SSH proxy jumping functionality, allowing connections to remote hosts via an existing SSH connection. This is useful for accessing hosts that are only reachable through an intermediate SSH server.

Changes:

• Added connect_via and connect_via_with_config methods to enable proxy jumping
• Added test infrastructure (second SSH server container) to validate the proxy jumping feature
• Added test case to verify proxy jumping works correctly

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/client.rs	Implements connect_via and connect_via_with_config methods for proxy jumping, plus test case
tests/docker-compose.yml	Adds second SSH server container (ssh-server-2) for proxy jumping tests
tests/async-ssh2-tokio/Dockerfile	Adds environment variables for second SSH server configuration
.env	Adds environment variables for second SSH server IP and port

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[Copilot]

Consider adding more comprehensive documentation for this new public API method, similar to the documentation for the connect method. The documentation should explain what happens when addr yields multiple addresses, how authentication works, and describe the parameters in detail. This would help users understand the proxy jumping feature better.