Expose runner limit error message on registration

What does this MR do?

This MR adds additional logic to decode and log the message present in the body of the /api/v4/runners response in the case where a Bad Request (400) status code is returned.

Why was this MR needed?

As noted in #27840, when we enabled the :ci_runner_limits FF globally some users started hitting the limits when registering a specific type of runner and were left wondering what had happened since the error message reported by gitlab-runner register is too generic (it just reported that "you may be having network problems"). When a plan limit is hit on GitLab, it returns an error in the body and a Bad Request (400) status code:

Runner type	HTTP response body
group	{"message":{"runner_namespaces.base":["Maximum number of ci registered group runners (50) exceeded"]}}
project	{"message":{"runner_projects.base":["Maximum number of ci registered project runners (50) exceeded"]}}

In this MR, we now treat the 400 error in a special way and try to decode the JSON payload. If we find any of the expected fields, we use that in the log message in the status field. For that, I needed to slightly alter network/client.go so that it accepts more than one status code to decide when to decode the JSON response. I also created a GitLabClient.doJSONMulti for the same purpose (which is now called by the previous GitLabClient.doJSON for compatibility).

What's the best way to test this MR?

1. On GitLab.com, create a new group named <your-user-name>-test-runner-limits.

2. Enable the :ci_runner_limits FF for that group. For that, go to #production and type:

   /chatops run feature set --group=<your-user-name>-test-runner-limits ci_runner_limits true

3. Build this branch:

   git switch pedropombeiro/27840/expose-runner-limit-error-message && make runner-bin-host

4. Register 51 runners on the new group:

   Assuming an environment variable GROUP_RUNNERS_REGISTRATION_TOKEN set to the runners registration token in your group's (https://gitlab.com/groups/your-username-test-runner-limits/-/settings/ci_cd) Runner registration page:

   for i in $(seq 1 51); do \
       out/binaries/gitlab-runner register -config /tmp/config.test.toml \
               --non-interactive \
               --executor "shell" \
               --url "https://gitlab.com/" \
               --description "group test runner" \
               --tag-list "shell,gdk,mac,test" \
               --run-untagged="false" \
               --locked="false" \
               --access-level="not_protected" \
               --registration-token="$GROUP_RUNNERS_REGISTRATION_TOKEN"; \
   done

   Upon registering the 51st runner (assuming your account is on the free plan) you should see the following error message:

   

5. On GitLab.com, create a new project named test-project-runner-limits under the group you created earlier.

6. Enable the :ci_runner_limits FF for that project. For that, go to #production and type:

   /chatops run feature set --project=<your-user-name>-test-runner-limits/test-project-runner-limits ci_runner_limits true

7. Register 51 runners on the new project:

   Assuming an environment variable PROJECT_RUNNERS_REGISTRATION_TOKEN set to the runners registration token in your project's (https://gitlab.com/your-username-test-runner-limits/test-project-runner-limits/-/settings/ci_cd) Runner registration page:

   for i in $(seq 1 51); do \
       out/binaries/gitlab-runner register -config /tmp/config.test.toml \
               --non-interactive \
               --executor "shell" \
               --url "https://gitlab.com/" \
               --description "group test runner" \
               --tag-list "shell,gdk,mac,test" \
               --run-untagged="false" \
               --locked="false" \
               --access-level="not_protected" \
               --registration-token="$PROJECT_RUNNERS_REGISTRATION_TOKEN"; \
   done

   Upon registering the 51st runner (assuming your account is on the free plan) you should see the following error message:

   

8. Unregister all runners from this MR:

   out/binaries/gitlab-runner unregister --all-runners --config /tmp/config.test.toml

What are the relevant issue numbers?

Closes #27840