Add the ability to include security contexts in container level for cluster checks runners

[mrdoggopat]

What this PR does / why we need it:

Which issue this PR fixes

• CONS-6803
• [BUG] - clusterchecks securityContext defined in Values not being applied datadog-agent#30676

Special notes for your reviewer:

Test plan:
Use the following configuration in your values.yaml:

clusterChecksRunner:
  enabled: true
  containers:
    agent:
      securityContext:
        runAsUser: 100
        readOnlyRootFilesystem: true
        allowPrivilegeEscalation: false
        capabilities:
          drop:
            - ALL
    initContainers:
      securityContext:
        runAsUser: 100
        readOnlyRootFilesystem: true
        allowPrivilegeEscalation: false
        capabilities:
          drop:
            - ALL

• Run kubectl get pod <cluster check pod> -o jsonpath='{.spec.containers[*].securityContext}' | jq . and double check that the security context are appropriately configured in the agent container.

The result should be:

{
  "allowPrivilegeEscalation": false,
  "capabilities": {
    "drop": [
      "ALL"
    ]
  },
  "readOnlyRootFilesystem": true,
  "runAsUser": 100
}

• Run kubectl get pod <cluster check pod> -o jsonpath='{.spec.initContainers[*].securityContext}' | jq . and double check that the security context are appropriately configured in the init containers.

The result should be the following for the init-config and init-volume init containers:

{
  "allowPrivilegeEscalation": false,
  "capabilities": {
    "drop": [
      "ALL"
    ]
  },
  "readOnlyRootFilesystem": true,
  "runAsUser": 100
}
{
  "allowPrivilegeEscalation": false,
  "capabilities": {
    "drop": [
      "ALL"
    ]
  },
  "readOnlyRootFilesystem": true,
  "runAsUser": 100
}

Also check if cluster checks work fine, can set the following to see if the KSM core check works fine as a cluster check

datadog:
  kubeStateMetricsCore:
    useClusterCheckRunners: true

...or use an integration check like Openmetrics.

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• Chart Version bumped
• Documentation has been updated with helm-docs (run: .github/helm-docs.sh)
• CHANGELOG.md has been updated
• Variables are documented in the README.md
• For Datadog Operator chart or value changes update the test baselines (run: make update-test-baselines)