Listen to `PosixSignalRegistration` for events

[bruno-garcia]

Different signals can be now received in .NET 6 through PosixSignalRegistration. We can rely on this to close a session, flush events, add breadcrumbs etc.

This task involves exploring which events are available, what are the limitations (allocations, timing, opening connections?) with regards to code we can run in there and come out with what we can build on top of it.

See: https://github.com/dotnet/runtime/issues/50527