Draft: feat: support multiple runners via runners.configs list

What does this MR do?

Adds a runners.configs list value as an alternative to runners.config, enabling multiple [[runners]] blocks to be registered from a single chart installation.

How it works

• runners.configs is a list of objects, each with a config key (the TOML template string, evaluated via tpl like runners.config) plus optional per-runner overrides
• The ConfigMap renders a separate config.template-{i}.toml per entry
• The register-the-runner script calls gitlab-runner register once per entry via a shared do_register helper function
• Per-runner flag overrides (name, maximumTimeout, runUntagged, protected) fall back to the top-level runners.* values when not set on the entry; an explicit false on a per-runner boolean wins over a top-level true
• runners.config (single string) continues to work exactly as before — no breaking change

Example

runners:
  configs:
    - config: |
        [[runners]]
          [runners.kubernetes]
            namespace = "{{ default .Release.Namespace .Values.runners.jobNamespace }}"
            image = "alpine"
      name: "alpine-runner"
      runUntagged: true
    - config: |
        [[runners]]
          [runners.kubernetes]
            namespace = "{{ default .Release.Namespace .Values.runners.jobNamespace }}"
            image = "ubuntu"
      name: "ubuntu-runner"
      protected: true

Why was this MR needed?

The chart previously only supported registering a single runner per installation. Users who needed multiple runners with different configurations (e.g. different images, tags, or executor settings) had to deploy multiple chart releases.

What's the best way to test this MR?

1. helm unittest -f 'tests/*.yaml' . — 14 new unit tests cover the feature; all 64 tests pass
2. Deploy with a runners.configs list and verify multiple runners appear in the GitLab UI after pod startup
3. Deploy with the existing runners.config and verify no behavior change (backward compat)

What are the relevant issue numbers?

N/A