Room stats API

[alan-george-lk]

This PR exposes a getStats() public API method to room.h to match Rust.

API design decisions

TL/DR: matches Rust SDK as 1:1 as possible.

I opted for the signature getStats() to match Rust: https://github.com/livekit/rust-sdks/blob/1848e77a32ca2f302a58323a7a21858165d81aad/livekit/src/room/mod.rs#L831

std::future -> async equivalent

The struct SessionStats also matches Rust: https://github.com/livekit/rust-sdks/blob/1848e77a32ca2f302a58323a7a21858165d81aad/livekit/src/rtc_engine/rtc_session.rs#L112

Stores vectors of publisher_stats and subscriber_stats to match Rust.

Error handling

• I opted to match the existing Track::getStats() which throws an exception

Testing

• Added unit test that checks the error path
• Added integration test that does some basic room operations and then obtains the stats

[stephen-derosa]

nice LGTM!