refactor: Update SortMergeJoin to use async spill abstractions and remove open_sync

[pantShrey]

Note: This PR depends on #21882 (pluggable SpillFile trait) and cannot be merged before it. Opening in parallel per @alamb's suggestion for easier review. The required SpillFile trait used here is defined in that base PR.To review locally, apply #21882 first and then stack this branch on top.

Which issue does this PR close?

• Depends on feat: introduce pluggable SpillFile trait and TempFileFactory for custom spill backends #21882, both prs together closes Allow pluggable file backends in DiskManager and IPCStreamWriter to support non-OS file systems #21215

Rationale for this change

materializing_stream.rs and bitwise_stream.rs were reading spilled batches via open_sync_reader / direct File::open calls, bypassing the SpillFile abstraction introduced in #21882. This PR migrates both to use SpillManager::read_spill_as_stream, allowing custom backends (Postgres BufFile, object storage) to handle spill reads without requiring an OS file path.

What changes are included in this PR?

• materializing_stream.rs: Eagerly restores spilled BufferedBatches via async streams before freezing, avoiding new state machine variants.
• bitwise_stream.rs: Replaces sync reads with an async poll_next_unpin loop, caching the stream to survive Poll::Pending.
• spill_file.rs: Removes open_sync_reader from the SpillFile trait (no longer needed).

Are these changes tested?

Covered by existing SMJ tests. No new tests added, the behavioral change is internal (sync → async IO path), observable only through custom backends which are not yet in tree.

Are there any user-facing changes?

No. Removes open_sync_reader from the SpillFile trait, this is a breaking API change for anyone implementing the trait, but the trait was introduced in #21882 which has not merged yet so there are no external implementors.