wall initial implementation

[cmoibosslady]

This pull request introduces an implementation of the 'wall' utility. It aims to be a first-draft binary crate since the flags of the wall command are not implemented yet.

New utility implementation:

• adds a new binary crate uu_wall on src/uu/wall with Cargo.toml specifying dependencies (uucore, clap, nix...)
• implements the core logic inside wall.rs. supports message input from stdin or command-line arguments
• a simple entrypoint in main.rs to call the utility with the uucore macro

CLI and mechanics:

• uses clap for parsing arguments. CLI flags are parsed but not yet implemented (placeholder for future extension)
• after retreiving the messages, scan all logged-in users via the utmpx utility and then proceed to write into their terminal the message.

[sylvestre]

nice
could you please add unit tests in tests/ ?
thanks