feat(client): add ClientBuilder::happy_eyeballs_timeout option

[jeffyaw]

feat(client): add ClientBuilder::happy_eyeballs_timeout option

What

Exposes hyper-util's HttpConnector::set_happy_eyeballs_timeout on both the
async and blocking ClientBuilder:

let client = reqwest::Client::builder()
    .happy_eyeballs_timeout(Some(Duration::from_millis(50)))
    .build()?;

None disables the parallel fallback entirely (addresses are tried
sequentially); the default stays at hyper-util's 300ms, so behavior is
unchanged for existing users. The plumbing follows the exact shape of
tcp_user_timeout (#2724) and the TCP keepalive options (#2675): a Config
field in src/async_impl/client.rs, a setter on ConnectorBuilder in
src/connect.rs that forwards to the underlying HttpConnector in all three
TLS variants, and a with_inner mirror in src/blocking/client.rs.

Why

hyper-util's 300ms default is a real, measurable tax on dual-stack hosts
where the preferred family does not answer:

• Concrete case, measured: a dual-stack localhost where the server is bound
  only to ::1. The IPv4 attempt is preferred and never completes, so the
  client waits out the full 300ms before trying ::1, which then connects
  immediately. Node, whose equivalent knob
  (autoSelectFamilyAttemptTimeout) defaults to 250ms and is user-settable,
  does the same request in ~15ms. Once per fresh connection.
• The fallback is already error-driven, so this only bites when the preferred
  family hangs rather than refusing -- but that is exactly the case a user
  cannot work around from outside the connector.

Related: #1318 ("Happy Eyeballs support", closed 2021) — that report turned
out to be an environment with no IPv6 routes at all, but the thread also
asked how to reach set_happy_eyeballs_timeout from reqwest, which is still
not possible today.

Since hyper-util already implements the timeout and its setter, reqwest just
needs to pass it through -- no new connection logic.

Non-goals

The HTTP/3 connector (src/async_impl/h3_client/connect.rs) has its own
separate happy-eyeballs implementation by design and is deliberately not
touched here; this option configures the TCP (HttpConnector) path only.