Improve echoserver example, bump Rust toolchain to 1.81

[JuliDi]

While trying out the echoserver, I've added some error handling and made the session closable with ctrl+c, which might often be what you want. So I thought upstreaming these changes might be a good thing.

While on it, I took the liberty of bumping the rust toolchain to 1.81, which does not seem to break anything.

[JuliDi]

There is still a clippy lint regarding unused struct fields:

warning: fields `key` and `algo` are never read
   --> russh/src/auth.rs:140:9
    |
139 |     PublicKey {
    |     --------- fields in this variant
140 |         key: CryptoVec,
    |         ^^^
141 |         algo: CryptoVec,
    |         ^^^^
    |
    = note: `CurrentRequest` has a derived impl for the trait `Debug`, but this is intentionally ignored during dead code analysis
    = note: `#[warn(dead_code)]` on by default

Not sure what that struct is used for and how to fix it. Maybe allow dead_code in there? Or prefix with an underscore?

Regarding rustfmt: should we use a nightly toolchain to run rustfmt? There are still some warnings about nightly-only features.

[Eugeny]

Thanks! Yes, the nightly rustfmt is still needed for imports_granularity etc.

[Eugeny]

@all-contributors add @JuliDi for code

[allcontributors[bot]]

@Eugeny

I've put up a pull request to add @JuliDi! :tada: