Improving near-sandbox-util API

[ghost]

Issue

In the README there's documentation to be added for the Rust section: https://github.com/near/sandbox/blob/50d8df010b8b436bc71ec1fbf9ed2841e5b131a1/README.md?plain=1#L31

Possible Solution

I jumped the gun and started on an implementation but it's better discussed.

We can add this missing documentation and also provide better UX for developers who will/might use the sandbox standalone. Using an api like proposed, near/near-workspaces-rs#88 (comment), will be an improvement over calling individual functions.
Below is my (rough) api proposal.

#[must_use = "init() must be called before running the sandbox"]
pub struct SandboxHandle {
    bin_path: PathBuf,
    home_dir: PathBuf,
    proc: Option<tokio::process::Child>,
}

impl SandboxHandle {
    fn new(version: &str, home_dir: &str) -> Result<()> { todo!() }

    fn init(&self) -> Result<()> { todo!() }

    fn run(&self) -> Result<()> { todo!() }

    fn run_with_args(&self, args: &[&str]) -> Result<()> { todo!() }
}

impl Default for SandboxHandle { ... }