Add Serve::with_executor and Executor trait for custom task spawning

[yulnr]

Closes #3703

Motivation

axum::serve hardcodes TokioExecutor for spawning connection tasks and hyper's internal HTTP/2 tasks. This is the one thing that cannot be customized by wrapping axum's API from the outside, users are forced to reimplement the entire serve loop (~150 lines) just to swap the executor.

This is needed for use cases like runtime telemetry (e.g. dial9-tokio-telemetry) where spawned tasks must be wrapped to capture scheduling delays and wake events.

Solution

• Add axum::serve::Executor trait for custom task spawning
• Add Serve::with_executor() and WithGracefulShutdown::with_executor() builder methods that take an Executor

Default remains TokioExecutor so it's fully backward compatible.

Design

We define a new axum::serve::Executor trait rather than reusinghyper::rt::Executor<Fut> directly, because the latter is generic at the trait level and we can't bound E to cover all the internal future types hyper needs. The new trait uses a generic method instead, and a thin HyperExecutor<E> adapter bridges to hyper internally.

Testing

• Existing serve tests pass
• New serving_with_custom_executor end-to-end test with duplex IO
• Compile test covers with_executor in both orderings (before and after with_graceful_shutdown)
• Doc examples compile and run