Support profiling existing process via --pid on macOS

[mhansen]

Would be lovely to be able to profile a slow app without having to reboot it. Just thought I might file the request to track it and provide a place to discuss possible solutions.

I see this was done for Linux in #56.

I understand there's some hooking of dynamic libraries (DYLD_INSERT_LIBRARIES) that happens pre-launch on macOS that probably makes this hard: https://github.com/mstange/samply?tab=readme-ov-file#known-issues

Thanks for Samply. It's cool, and I enjoy using it!