Nc/1oct/py prepare multipart (#1054)

- Add requests-toolbelt dep
- Use custom http adapter
- subclass so that we can override blocksize (default one too small to
maximize performance on modern network stacks)
- override default pool size to match the max nr of batch tarcing
threads (otherwise some threads would be left waiting for connections to
be available)
- actually mount the adapter (previous behavior was never mounting the
adapter because sessions always have http and https adapters)
- note before retry policy was being ignored, and no transport level
retries were being done
- Add attachments field to run dict/model
  - update _run_transform to collect/drop attachments
- warn if attachments are passed to an endpoint that can't accept them,
in this case inject runs without attachments