Add a public API to read call stacks in EventPipeEventSource

EventPipeEventSource writes call stacks into the trace event eventRecord internal field (here).

There is no public API to read the call stacks from the eventRecord, so call stacks are not accessible, particularly when using the "push" model with EventPipeEventSource. The only public way the read call stacks is to use TraceLog, which reads the eventRecord (here).

Would it be possible to add a public API to read the call stacks from the events generated by EventPipeEventSource?

API example:

public class EventPipeStack
{
    public EventPipeStack(ulong[] addresses);

    public int Length { get; }

    public ulong this[int index] { get; }
}

public class EventPipeEventSource
{
    // Reads call stack from `eventRecord`
    public static bool TryGetStack(TraceEvent traceEvent, out EventPipeStack stack);
}

Of course, there are many design options for the API. I simply thought that adding TryGetStack in EventPipeEventSource next to GetEventRecordForEventData would make sense.

Review EventPipeEventSource.cs around the eventRecord handling and the GetEventRecordForEventData area, then compare how TraceLog.cs reads call stacks. Determine a public API shape that works for EventPipeEventSource push-model events; done means callers can reliably retrieve the call-stack addresses from the generated TraceEvent.