perf(blob): parallelize descriptor range reads

[JingsongLi]

What changed

• Resolve BlobDescriptor columns and URI range groups concurrently.
• Share a reader-lifetime limiter across metadata and range I/O.
• Cap active I/O at 8 requests and range admission at 64 MiB.
• Preserve row and column order, with deterministic concurrency tests.

Why

External BlobDescriptor materialization awaited descriptor columns, URI groups, and merged ranges serially. On high-latency object storage, those round trips accumulated across a batch. This change adds bounded asynchronous concurrency while retaining the existing per-URI range coalescing.

Impact

This improves latency and throughput for batches containing multiple non-coalescible external descriptors while bounding request pressure and admitted range bytes. The ordinary .blob reader and public configuration remain unchanged.

Validation

• cargo test -p paimon
• cargo clippy -p paimon --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check

[leaves12138]

The latest update resolves the Clippy failure. I rechecked the bounded parallel descriptor-read implementation and the new concurrency coverage, and found no correctness issues. The targeted overlap test and formatting checks pass locally.