export: Add a streamed way of getting room keys

[andybalaam]

This is part of my work on reducing the memory usage of exporting all our room keys #2914.

It's small part: we offer a streamed method on OlmMachine called export_room_keys_stream that is similar to export_room_keys except it returns a Stream, meaning the caller does not need to have all the keys they requested in memory at the same time.

Inside its implementation, it uses CryptoStore::get_inbound_group_sessions, which returns a Vec, so it doesn't (yet) prevent unbounded memory use.

In a test I did with 10K keys, when I used this method and serialised the result directly to JSON without holding it in an intermediate Vec I saw a few MB of memory savings in Element Web during export.

So there is some immediate benefit, but possibly more importantly, this just feels like the right interface to offer: especially when we are exporting all keys, we don't want to be forced to hold them all in a Vec as export_room_keys does. In future, some PR like #3060 will allow us to stream properly from the database, and not hold all keys in an intermediate Vec as we do now.

I plan to follow this up with a PR in matrix-rust-sdk-crypto-wasm that realises the memory saving I referred to above.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f0354d1) 83.85% compared to head (496ce6b) 83.86%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3144   +/-   ##
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  Coverage   83.85%   83.86%           
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  Files         231      231           
  Lines       23856    23860    +4     
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+ Hits        20005    20009    +4     
  Misses       3851     3851           

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[poljar]

Let's avoid adding more methods to the OlmMachine especially if they are clearly storage related for which we already expose the OlmMachine::store().