Add functions for incoming multi-stream connections

[tomaka]

They're unused in practice, but there's no reason to not allow ingoing multi-stream connections in the underlying state machine.

cc #2802

[mergify]

Automatically approving tomaka's pull requests. This auto-approval will be removed once more maintainers are active.

[github-actions]

twiggy diff report

Difference in .wasm size before and after this pull request.

---

 Delta Bytes │ Item
─────────────┼──────────────────────────────────────────────────────────────────────────────────────────────────────────
        +777 ┊ smoldot::libp2p::collection::multi_stream::MultiStreamConnectionTask<TNow,TSubId>::new::h0ec5113094ca96e9
        -716 ┊ smoldot::libp2p::collection::multi_stream::MultiStreamConnectionTask<TNow,TSubId>::new::h9c7573e5fb7e11bb
        +392 ┊ smoldot::libp2p::collection::Network<TConn,TNow>::insert_multi_stream::ha6f96bfaae673f10
        -390 ┊ smoldot::libp2p::collection::Network<TConn,TNow>::insert_multi_stream::hadd4facec4179ba9
         +16 ┊ type[67]: (i32, i32, i32, f64, i32, i32, i32, i32, i32, i32, i32, i32, i32) -> nil
         -15 ┊ type[67]: (i32, i32, f64, i32, i32, i32, i32, i32, i32, i32, i32, i32) -> nil
         -10 ┊ type[71]: (i32, i64, i32, f64, i32, i32) -> i64
         +10 ┊ type[72]: (i32, i64, i32, f64, i32, i32) -> i64
         -10 ┊ type[76]: (i32, i32, i32, i32, f64, i32) -> i64
         +10 ┊ type[77]: (i32, i32, i32, i32, f64, i32) -> i64
          +9 ┊ type[71]: (i32, i32, f64, i32, i32, i32) -> nil
          -9 ┊ type[72]: (i32, i32, f64, i32, i32, i64) -> nil
          +9 ┊ type[73]: (i32, i32, f64, i32, i32, i64) -> nil
          -9 ┊ type[78]: (i32, i32, f64, i32, i32, i32) -> nil
          -8 ┊ type[74]: (i32, f64, i32, i32, i32) -> nil
          +8 ┊ type[75]: (i32, f64, i32, i32, i32) -> nil
          -8 ┊ type[75]: (i32, i32, i64, i64, i32) -> nil
          +8 ┊ type[76]: (i32, i32, i64, i64, i32) -> nil
          -8 ┊ type[77]: (i32, i32, i32, f64, i32) -> nil
          +8 ┊ type[78]: (i32, i32, i32, f64, i32) -> nil
          +2 ┊ ... and 3 more.
         +66 ┊ Σ [23 Total Rows]