perf: Jobify SendBatchedMessages in UnityTransport [MTT-4999]

[simon-lemay-unity]

This PR jobifies the SendBatchedMessages method in UnityTransport. The main goal of this operation is just to move the BeginSend and EndSend calls into Burst-compiled code, since otherwise they end up generating GC allocations when invoking their Burst function pointers.

To demonstrate that this results in a performance gain, I profiled a simple Boss Room play session. Here's a representative frame without the changes:

And here's a similar frame with the changes applied:

As can be observed, GC allocations are completely gone from SendBatchedMessages and its runtime is also reduced.

There's still much more that we could do to take advantage of jobs here. For example, we could parallelize the sends for all the targets (although we should profile that; the cost of scheduling these jobs may be more than the savings we'd get). But for now I went with the simplest change that moved us forward.

This PR also modifies the ErrorUtilities to be usable from within Burst. While modifying that, I also removed support for a bunch of error codes that simply can't be returned here (either because our APIs don't return them anymore, or because the usage patterns in UnityTransport make them impossible to encounter).

Changelog

N/A (Didn't think it worthy of a changelog entry. Let me know if you feel otherwise.)

Testing and Documentation

• No tests have been added (but most existing tests run the new code).
• No documentation changes or additions were necessary.