Tezt_cloud: Add multiple backends option

What

Now that we can have multiple backends for PPX profiling (!18373 (merged)), we want to propagate this change to tezt-cloud.

Why

One reason would be to check the OpenTelemetry results of our DAL experiments.

How

If the --ppx-profiling option is set, then by default we have the txt and json formats of the profiling activated. If, besides that, we also have prometheus and opentelemtry activated from the CLI, then their respective backends will also be added to the PROFILING_BACKENDS environment variable list.

Manually testing the MR

Using tezt-cloud with --ppx-profiling and any combination of prometheus and opentelemetry CLI arguments activated.

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