More generic profiler support (eflambe in particular)

I saw a cool talk at Code BEAM EU about [performance optimizations and the 1 billion row challenge](https://codebeameurope.com/talks/the-one-billion-row-challenge-in-elixir-from-12-minutes-to-25-seconds/) by @rajrajhans and he used [eflambe](https://github.com/Stratus3D/eflambe) for profiling.

So the question was... why not add support for more profilers? Right now we only support built-in profilers.

In true benchee mannerism though I'd want it to be more generic:
* no hard dependency on eflambe, most likely not even an optional one - point to a plugin
* since it's a plugin, we'd need to design a behavior (just like for formatters) to conform with for "profiling plugins"

likely similiar to the interface of the built-in profilers which we already support: https://hexdocs.pm/mix/Mix.Tasks.Profile.Tprof.html

I'd expect that interface to be quite simple, but I'm not looking deeply into it just now :) Here is where/how we currently use them: https://github.com/bencheeorg/benchee/blob/main/lib/benchee/profile.ex#L109

https://github.com/bencheeorg/benchee/blob/main/lib/benchee/benchmark/collect/profile.ex

It boils down to:

```elixir
return_value = profiler_module.profile(function, profiler_opts)
```

With that we could also remove our reliance on the mix tasks (I'm unsure how much benefit they give us over calling the profilers themselves).

Start by reading lib/benchee/profile.ex around line 109 and lib/benchee/benchmark/collect/profile.ex, then compare the existing profiler interface with Mix.Tasks.Profile.Tprof. Define the profiling-plugin behavior and its integration points. Done means profilers can be supplied through a generic plugin interface without a hard eflambe dependency or reliance on Mix tasks.

Written by the indexing model from the issue text.