Rebases onto apache#687's executor pool support per @wgtmac's review:

* FileCleanupStrategy now takes an OptionalExecutor in its constructor.
  When the custom DeleteWith() callback is configured, per-path deletes
  fan out through a TaskGroup that uses the executor (or runs serially
  when none is set, preserving prior behavior).
* The FileIO bulk delete path wraps file_io_->DeleteFiles in a tight
  RetryRunner<retry::StopRetryOn<kNotFound>> so transient FileIO errors
  no longer give up after the first attempt. Mirrors Java's
  Tasks.foreach(...).stopRetryOn(NotFoundException.class).retry(N).
* Adds ExpireSnapshots::Executor(OptionalExecutor) so callers can opt
  in to parallel deletion, and threads it down through both
  IncrementalFileCleanup and ReachableFileCleanup.
* Drops the std::async / std::thread / std::span machinery and the
  ad-hoc retry loop -- replaced by util/task_group.h and
  util/retry_util.h from apache#687.

Adds ExecutorDispatchesDeletesConcurrently test that wires a
test::ThreadExecutor through the new API and asserts the executor
received one submission per file.

Rebases onto apache#687's executor pool support per @wgtmac's review:

* FileCleanupStrategy now takes an OptionalExecutor in its constructor.
  When the custom DeleteWith() callback is configured, per-path deletes
  fan out through a TaskGroup that uses the executor (or runs serially
  when none is set, preserving prior behavior).
* The FileIO bulk delete path wraps file_io_->DeleteFiles in a tight
  RetryRunner<retry::StopRetryOn<kNotFound>> so transient FileIO errors
  no longer give up after the first attempt. Mirrors Java's
  Tasks.foreach(...).stopRetryOn(NotFoundException.class).retry(N).
* Adds ExpireSnapshots::Executor(OptionalExecutor) so callers can opt
  in to parallel deletion, and threads it down through both
  IncrementalFileCleanup and ReachableFileCleanup.
* Drops the std::async / std::thread / std::span machinery and the
  ad-hoc retry loop -- replaced by util/task_group.h and
  util/retry_util.h from apache#687.

Adds ExecutorDispatchesDeletesConcurrently test that wires a
test::ThreadExecutor through the new API and asserts the executor
received one submission per file.

Rebases onto apache#687's executor pool support per @wgtmac's review:

* FileCleanupStrategy now takes an OptionalExecutor in its constructor.
  When the custom DeleteWith() callback is configured, per-path deletes
  fan out through a TaskGroup that uses the executor (or runs serially
  when none is set, preserving prior behavior).
* The FileIO bulk delete path wraps file_io_->DeleteFiles in a tight
  RetryRunner<retry::StopRetryOn<kNotFound>> so transient FileIO errors
  no longer give up after the first attempt. Mirrors Java's
  Tasks.foreach(...).stopRetryOn(NotFoundException.class).retry(N).
* Adds ExpireSnapshots::Executor(OptionalExecutor) so callers can opt
  in to parallel deletion, and threads it down through both
  IncrementalFileCleanup and ReachableFileCleanup.
* Drops the std::async / std::thread / std::span machinery and the
  ad-hoc retry loop -- replaced by util/task_group.h and
  util/retry_util.h from apache#687.

Adds ExecutorDispatchesDeletesConcurrently test that wires a
test::ThreadExecutor through the new API and asserts the executor
received one submission per file.

…649)

### What

* `FileCleanupStrategy` now takes an `OptionalExecutor` in its
constructor. When `DeleteWith()` is configured, per-path deletes fan out
through `TaskGroup<>` -- uses the supplied executor when set, otherwise
runs the callbacks serially on the calling thread via
`RunTasksSingleThreaded`. Preserves prior single-threaded behavior by
default.
* The `FileIO::DeleteFiles` bulk path is wrapped in
`RetryRunner<retry::StopRetryOn<ErrorKind::kNotFound>>` with a tight
budget (2 retries, 100ms-1s backoff, 5s total). Mirrors Java's
`Tasks.foreach(...).stopRetryOn(NotFoundException.class).retry(N)`. The
retry primarily helps atomic-bulk FileIO impls (e.g. an S3
`DeleteObjects`-backed FileIO) ride out transient throttles --
best-effort, see code comment for the fail-fast-iterative caveat.
* New public builder
`ExpireSnapshots::ExecuteDeleteWith(OptionalExecutor)` (named after
Java's `executeDeleteWith(ExecutorService)`) so callers opt in to
parallel deletion; threaded through both `IncrementalFileCleanup` and
`ReachableFileCleanup`.
* `DeleteWith()` doc note clarifies that the user-supplied callback may
be invoked concurrently from worker threads and must be thread-safe.
* Drops the `std::async` / `std::thread` / `std::span` machinery and the
ad-hoc retry loop -- replaced by `util/task_group.h` and
`util/retry_util.h` from #687.

### Test coverage

Existing 25 `ExpireSnapshots*` tests continue to pass unchanged (they
hit the default no-executor path).

Adds `ExpireSnapshotsCleanupTest.ExecutorDispatchesDeletesConcurrently`
-- wires a `test::ThreadExecutor` through `ExecuteDeleteWith()`, runs an
`ExpireSnapshotId` cleanup with a custom `DeleteWith` callback, and
asserts the executor received one submission per file (data + manifest +
manifest-list = 3) and that all paths show up in the mutex-guarded sink.