Support Pod DNS Config and Policy for Kubernetes executor

What does this MR do?

Adds support for Pod DNS Config.

Why was this MR needed?

#6562 (closed)

What's the best way to test this MR?

There are 2 new tests covering the parsing of the new options:

go test -count=1 -timeout=45s -v -run TestSetupBuildPod ./executors/kubernetes

Manual test

1. Register a kubernetes runner and then merge the following with the executor configuration (note that we disable the default DNS policy so that we can add our own config):

   [[runners]]
     name = "kubernetes-runner"
     url = "https://gitlab.com/"
     executor = "kubernetes"
     [runners.kubernetes]
       dns_policy = "none"
       [runners.kubernetes.dns_config]
         nameservers = ["192.168.6.254"]
         options = [{ name = "single-request-reopen" }]

2. Start the runner.

3. Create a .gitlab-ci.yml file containing the following:

   start_evaluation:
     script:
       - cat /etc/resolv.conf
     tags: [kubernetes]

4. The output of the CI job should be the following:

   

Without this change, the output would be:

What are the relevant issue numbers?

Closes #6562 (closed)