[EventPipe] Block EventPipeProvider Deletion for ongoing callbacks

[mdh1418]

Fixes #80666

This PR aims to align behavior between EventPipe's Unregister logic and ETW's Unregister logic by blocking EventPipe's DeleteProvider for in-flight callbacks, so that the gchandle will not be freed before the callback completes. (ETW has its own lock for ETW commands/callbacks).

Our initial attempt to add a corresponding EventPipe lock revealed to us that locks should not be taken around the callback (specifically performing the callback within a lock) because it breaks concurrent callbacks scenarios.

In this PR, we track the EventPipeProvider's callbacks that have been prepared but not yet invoked (i.e. in-flight callbacks), and leverage a signal set/wait to block the EventPipe Provider's deferred deletion.

---

Repro

Reproduced the crash by:

Console.WriteLine("MIHW Ready to create TestEventSource");
Console.ReadKey();
var testEventSource = new TestEventSource();
Console.WriteLine("MIHW Ready to dispose TestEventSource");
Console.ReadKey();
testEventSource.Dispose();
Console.WriteLine($"MIHW TestEventSource disposed.");

[EventSource(Name = "TestEventSource")]
public class TestEventSource : EventSource
...

1. Launching the above sample app in windbg
2. Connecting an EventPipe session via dotnet-trace collect --providers TestEventSource -p <pid of app from dotnet-trace ps>
3. Setting a breakpoint in ep-provider.c provider_invoke_callback at callback invocation
4. Closing the EventPipe session (enter || ctrl + c) to hit that breakpoint and freezing the thread
5. Continuing the application logic by disposing the EventSource
6. Unfreezing the previously frozen thread and continuing

Resulted in a NullReferenceException crash.

Testing

Performed the same steps as above with the changes in this PR, Dispose is blocked until the callback completes.

[jkotas]

Can you re-enable the disabled test?

[jkotas]

The change LGTM, but I am not deeply familiar with EventPipe code.

[jkotas]

/azp run runtime-coreclr outerloop

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Azure Pipelines successfully started running 1 pipeline(s).

[noahfalk]

Few comments inline but mostly looks good!

[mdh1418]

/azp run runtime-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mdh1418]

/azp run runtime-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mdh1418]

/ba-g The failing tests uncaught by build analysis are #104905 and #103347, not sure why build analysis didn't recognize the match in 103347.