Implement span attachments in streaming mode

I haven't found a single hit for `add_attachment(add_to_transactions=True)` anywhere on GitHub, so I don't believe this has super high priority, but we should add it at some point.

[https://develop.sentry.dev/sdk/telemetry/spans/span-protocol/#span-attachments]()

Start by reading the linked span attachments protocol and searching the repository for add_attachment(add_to_transactions=True) and streaming-mode span handling. Determine the existing transaction and span attachment paths, then verify that attachments are included correctly when spans are streamed according to the protocol.

Written by the indexing model from the issue text.