PLZ: refine env vars and add new CLI arguments

[yorugac]

Feature Description

Current definition and capabilities of PLZ test runs are minimal in comparison to public GCk6 tests. We're going to polish it a bit, so that PLZ becomes more similar to other kinds of tests. Specifically:

1. The .spec.arguments definition should be expanded with some additional flags to k6 CLI: --no-summary, --verbose (TODO: clarify this one)
2. User-defined env vars must be passed with -e CLI flag
3. The call to /loadtests/v4/test_runs(%s)?$select=id,run_status,k8s_load_zones_config,k6_runtime_config must also include ,load_zone_distribution and the format of expected response is now this:

   {
     "k8s_load_zones_config": {
       "instance_count": <int>,
       "k6_archive_temp_public_url": <str>,
       "load_runner_image": <str>,
   
       "environment": <map>, # user-defined
   
       "cli_flags": { # user-defined
         "tags": <map>,
         "user_agent": <str>,
         "blacklist_ips": [<str>], 
         "blocked_hostnames": [<str>],
         "include_system_env_variables": bool,
       },
       "gck6_env_vars": { # defined by GCk6
         "K6_CLOUD_TOKEN": "token",
         "K6_LOG_OUTPUT": "value", 
       },
     },
     load_zone_distribution: {
           "label": {"loadZone": <str>, "percent": <int>}
     }

4. cli_flags must all be passed to the .spec.arguments as k6 tags, --user-agent, --blacklist-ip and --block-hostnames CLI args respectively.
5. gck6_env_vars must be set as env vars for the runner
   • NOTE: K6_LOG_OUTPUT format is pending; we need to ensure we don't have a degradation here.
6. As described here, we need to set 4 additional env vars as part of user-defined env vars, via -e tag.
   • three of them can be set as part of .spec.arguments: K6_CLOUDRUN_LOAD_ZONE (taken from distribution field), K6_CLOUDRUN_DISTRIBUTION (also from distribution field?), K6_CLOUDRUN_TEST_RUN_ID.
     • Additionally pass load zone value as "load_zone" tag to .spec.arguments.
   • the 4th env var, K6_CLOUDRUN_INSTANCE_ID must be set at the level of k6 runner definition, because its value is not known until then. Consider the best implementation there + potential refactoring.
   • Theses vars must be added on top of user-defined env vars: they have higher priority and should overwrite the values from the user if any.

Major change: K6_CLOUD_TOKEN added in the endpoint brings a new type of token to a PLZ - a test run specific token.

Suggested Solution (optional)

Most is self-explanatory. But as mentioned, there might also be small ways to improve / refactor some bits of code on the way.

Additionally, change with the token must be confirmed to be non-breaking for PLZ, as there wasn't a hard guarantee of it being non-breaking from GCk6 side.

Already existing or connected issues / PRs (optional)

No response