Add RuntimeHandle support for Tokio runtime segregation

[alexanderbianchi]

What Was Implemented

1. Core API (crates/iceberg/src/io/file_io.rs)

Added RuntimeHandle type that wraps a tokio::runtime::Handle:

#[derive(Clone, Debug)]
pub struct RuntimeHandle(pub tokio::runtime::Handle);

impl RuntimeHandle {
    pub fn new(handle: tokio::runtime::Handle) -> Self;
    pub fn current() -> Self;
}

How It Works

1. User configures runtime handle:

   let file_io = FileIOBuilder::new("s3")
       .with_extension(RuntimeHandle::new(io_handle))
       .build()?;

2. iceberg-rust extracts handle during build:

   • Storage::build() gets RuntimeHandle from extensions
   • Creates CustomTokioExecutor wrapping the handle
   • Wraps in opendal::Executor

3. Executor applied to operators:

   • When create_operator() is called, executor is applied via update_executor()
   • All opendal async operations spawn tasks on the configured runtime