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
Result -> EngineResult 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

• The current version of this design has an error struct return type with a message and code. This code enum is "implied" from what went wrong, i.e. not returned from Rust FFI directly
• The Rust FFI only returns a string
• I am a bit conflicted about this Result<> design with the error code, as it adds 2 public types (struct/enum) and a whole header, for a code that doesn't actually exist other than in this SDK
• The alternative is a Result<SessionStats, std::string> which would pass along the error string on failure with no additional types beyond the actual data struct

Testing

• Added integration test that does some basic room operations and then obtains the stats

[stephen-derosa]

nice LGTM!

[stephen-derosa]

should we name getStatsAsync() so its clear this is async/matches the ffi call?