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.

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