Implement in-place update contracts

[alexander-demicev]

This is a tracking issue (epic) for in-place updates.

In-place updates were introduced in CAPI starting with v1.11.x. Since we have now bumped to v1.12.x, we can safely begin implementing this feature. Below is a high-level checklist of what needs to be done:

• Introduce a feature gate
  • Add a feature gate for enabling in-place updates.
• Implement contracts (using kubeadm as a reference)
  • Use the kubeadm implementation in CAPI as the primary reference, as it is currently the best and the only example.
  • Note CAPI maintainers had to refactor parts of the Kubeadm control plane provider to make this work correctly (optimizing the rollout process, ensuring maxSurge is respected, and eliminating race conditions).
  • Be prepared for this task to grow in complexity as similar refactoring may be required.
• Write a mock update extension for CAPRKE2
  • Implement a simple "mock" update extension for CAPRKE2.
  • Use the kubeadm CAPI extension as a reference.
• Add E2E coverage
  • Leverage the update extensin to write e2e tests.
• Update documentation
  • Document the new feature, configuration options, and expected behavior.