Inject kubectl-compatible config file into the CI job

What does this MR do?

Adds a KUBECONFIG predefined CI variable for use with the Kubernetes Agent CI Tunnel.

Instead of building a hash of configuration options (as we do currently for the legacy cluster integration), the config file is generated using small wrapper classes. This may seem like overkill, but it will allow for easier file generation in the future when we must merge Kubernetes contexts from multiple sources (Legacy cluster integration, Project and Group-level indexes).

How to setup and validate locally (strongly suggested)

1. Create an Agent configuration project
2. Register an agent for your project
3. Enable the agent_kubeconfig_ci_variable feature flag (Feature.enable(:agent_kubeconfig_ci_variable))
4. Run a CI job with the following template:

stages:
  - test

test:
  stage: test
  script:
    - cat "$KUBECONFIG"

You should see a config file similar to the following:

---
apiVersion: v1
kind: Config
clusters:
- name: gitlab
  cluster:
    server: https://kas.example.com/k8s-proxy
users:
- name: agent:137
  user:
    token: ci:137:[MASKED]
contexts:
- name: k8s-agent/agent-config:my-agent
  context:
    cluster: gitlab
    user: agent:137

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #324275 (closed)