[services][backup] PullBackup - fix tracing span

Summary:
Logging in async_stream wasn't displaying log spans correctly, this diff fixes that.
I needed to introduce a new dependency tracing-futures (official package from tracing vendor) because the core tracing::Instrumented<T> doesn't implement Stream<T>. With this dependency, we can use <impl Stream>::instrument() and <impl Stream>::in_current_span() which are required to correctly trace streaming logs.

Depends on D6246

Question: Should we display user_id in logs? Or is it sensitive data?

Test Plan:
Ran integration tests and ensured logs are displayed correctly

Reviewers: ashoat, varun, jon, michal

Reviewed By: jon

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6247