Add kubernetes runners allowPrivilegeEscalation security context configuration

What does this MR do?

Add kubernetes runners allowPrivilegeEscalation security context configuration

Why was this MR needed?

Provide the capability to configure the allowPrivilegeEscalation security context for the runner instances started on a kubernetes cluster that has OPA enabled:

ERROR: Job failed (system failure): prepare environment: admission webhook "validation.gatekeeper.sh" denied the request: [denied by psp-allow-privilege-escalation-container] Privilege escalation container is not allowed: build
[denied by psp-allow-privilege-escalation-container] Privilege escalation container is not allowed: helper. Check https://docs.gitlab.com/runner/shells/index.html#shell-profile-loading for more information

What's the best way to test this MR?

Try to start a job using k8s runners on a with OPA enabled with psp-allow-privilege-escalation-container:

• provide no allow_privilege_escalation value and the default will be determined by defaultAllowPrivilegeEscalation or True (as currently default in k8s) if nothing is defined (https://kubernetes.io/docs/concepts/policy/pod-security-policy/#privilege-escalation).
• provide allow_privilege_escalation=true value and the container start will be allowed
• provide allow_privilege_escalation=false value and the container start will be denied

What are the relevant issue numbers?

Implements #26998 (closed)