python(feat): add export api for sift client

[wei-qlu]

What was changed

Adds a high-level data export API to sift_client to reach parity with sift_py (soon to be deprecated).

• DataExportAPIAsync with a single export() method that accepts domain objects or raw string IDs for runs, assets, or time-range-based exports and returns a Job handle
• Job.wait_and_download() polls until complete, downloads results, and optionally extracts zip files (delegates to JobsAPIAsync)
• ExportsLowLevelClient wraps gRPC protobuf calls, the high-level client has zero proto imports
• Utility helpers added under _internal/util/:
  • channels.py - resolve_calculated_channels() resolves name-based channel identifiers to UUIDs via the channels API (the export API requires UUIDs)
  • file.py - download_file() streams downloads in 4 MiB chunks; extract_zip() extracts and cleans up archives
  • executor.py - run_sync_function() runs blocking calls off the event loop via run_in_executor
• Sync wrappers and type stubs registered. Wired up as client.data_export and client.async_.data_export

Verification

• Unit tests for input validation, domain object resolution, low-level delegation, wait/download status handling, and calculated channel resolution
• Integration tests for async and sync export paths

[nathan-sift]

This looks good, thanks for the changes! Approving, but it looks like you might need to re-update the stubs

[alexluck-sift]

lgtm! Nice work!