check-in v1: return feature flags for experimental experiments

[bassosimone]

In https://github.com/ooni/probe-cli/pull/1355, we introduced the concept of experimental experiments. These are OONI experiments that the engine considers disabled unless a bypass environment variable is set or check-in has instructed the probe that it is allowed to use these experiments. (We cache the check-in flags for 24h.)

This issue is about enabling the experimental experiments using the check-in flags. In practical terms, we need to extend the feature flags to unconditionally include the following feature flags:

// This JSON contains a subset of how the whole check-in response should look like
{
  "conf": {
    "features": {
      "torsf_enabled": true
    }
  }
}

I marked torsf as disabled because potentially upcoming SNI changes in snowflake may require an emergency release to fix the SNI we are using. However, for the time being it should be enabled via the check-in API.

[bassosimone]

We're good, let's close this issue as done!