[experimental] Ask copilot to check perf profile degradation (#146271)

We can optionally add copilot as a reviewer. As per
[documentation](https://docs.github.com/en/copilot/how-tos/agents/copilot-code-review/using-copilot-code-review#customizing-copilots-reviews-with-custom-instructions),
we can customize the review. This PR asks it to pay attention to the
kind of code that might corrupt performance data, or, in the case of
sample profiling, result in poor profiles by dropping debug info.

Author: mtrofin

.github/copilot-instructions.md

@@ -0,0 +1,4 @@
+When performing a code review, pay close attention to code modifying a function's
+control flow. Could the change result in the corruption of performance profile
+data? Could the change result in invalid debug information, in particular for
+branches and calls?