feat(pool): add connect_timeout option for spawned connection tasks

[mmustafasenoglu]

Description

This PR adds a new connect_timeout option to PoolOptions that, when set, spawns the connection attempt as a separate task in Pool::acquire(). This ensures that if acquire() is cancelled or times out, the connection attempt continues in the background rather than being aborted.

If the connection succeeds but no one picks it up (because acquire() was cancelled), the connection is automatically returned to the pool's idle queue.

Problem

Currently, acquire() wraps both the semaphore/idle-connection check and the actual connection creation in a single timeout. If acquire() is cancelled (e.g., due to its own timeout), the in-flight connect() call is also aborted. Under high contention, this can cause:

• Connection churn: connections are created and immediately discarded
• Pool instability: DecrementSizeGuard drops, pool size fluctuates
• Cascading timeouts: other waiting tasks also time out

This is the root cause behind several reported issues: #3315, #3132, #2848.

Solution

When connect_timeout is set on PoolOptions:

1. The connection attempt is spawned as a separate async task via crate::rt::spawn
2. The spawned task uses its own deadline based on connect_timeout
3. A futures_intrusive::channel::oneshot channel communicates the result back
4. If acquire() is cancelled (receiver dropped), the sender detects this and returns the connection to the pool's idle queue

When connect_timeout is not set (the default), the existing behavior is preserved exactly -- no breaking changes.

Changes

• sqlx-core/src/pool/options.rs: Added connect_timeout: Option<Duration> field, builder method, getter, and updated Debug/Clone impls
• sqlx-core/src/pool/inner.rs: Modified acquire() to spawn connection as a separate task when connect_timeout is set

Usage

use sqlx::postgres::PgPoolOptions;
use std::time::Duration;

let pool = PgPoolOptions::new()
    .max_connections(10)
    .acquire_timeout(Duration::from_secs(5))
    .connect_timeout(Duration::from_secs(10))  // NEW
    .connect("postgres://localhost/mydb")
    .await?;

Notes

• Uses futures_intrusive::channel::oneshot_channel for cross-runtime compatibility (works with tokio, async-std, smol)
• No new dependencies added
• Not a breaking change -- opt-in via new optional field
• Default behavior unchanged when connect_timeout is not set

Closes #3513

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