feat(client): expose connection reuse state to Connected

[zino299792]

• Add a field is_reused to Connected and a method exposing is_reused.
• Add a new function mark_as_reused in the Poolable interface that allows to mark a connection as reused.
• Mark a connection as reused in Pool<T, K>::reuse via Pollable::mark_as_reused.
• Tests
  • src/client/legacy/pool.rs: test dumb connection CanMarkReused
  • src/tests/legacy_client.rs: test capturing connection reuse info with hyper_util::client::legacy::connect::capture_connection.

Fix #3835.

[zino299792]

Test results:

$ cargo test reuse --features full

running 1 test
test client::legacy::pool::tests::mark_conn_as_reused ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 38 filtered out; finished in 0.00s

     Running tests/legacy_client.rs (target/debug/deps/legacy_client-77b3265b9280e812)

running 1 test
test get_conn_reuse_info_via_connected ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 17 filtered out; finished in 0.05s

[zino299792]

Hi @seanmonstar, if you have time, could you take a look on this PR?