Tezt/Cloud: allow restricting which backend to use

What

Allows choosing which profiler backends to use for a given scenario

Why

Now that the profiler can be started on multiple backends and tezt-cloud infers which to start from the CLI, we also want the ability to restrict which to actually use: using Prometheus in general is useful, but we don't necessarily want its profiler backend. Similarly, it can be useful to only start the Opentelemetry profiler instead of the whole stack.

How

Add a new option --ppx-profiler-backends to select the profilers. It will defaut to !18373 (merged) selection without the option (or if no backend is given)

Manually testing the MR

Run a tezt-cloud scenario with --ppx-profiling --ppx-profiler-backend txt for example, you shouldn't have the json report outputed. You can also test with only opentelemetry from MR !XXXXX, and witness the result of the profiling in the Jaeger instance.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR