Make Plan id and timestamp fields optional

[nekwar]

Description

Fixes https://github.com/k0sproject/k0s/issues/4058

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update

How Has This Been Tested?

• [x] Manual test
• [ ] Auto test added

k0s cluster was created with newly built binary, Plan without spec.id and spec.timestamp was created:

kind: Plan
metadata:
  name: autopilot
spec:
  commands:
    - k0supdate:
        version: v1.29.1+k0s.1
        platforms:
          linux-amd64:
            url: https://github.com/k0sproject/k0s/releases/download/v1.29.1+k0s.1/k0s-v1.29.1+k0s.1-amd64
        targets:
          controllers:
            discovery:
              selector:
                labels: app=123

Checklist:

• [x] My code follows the style guidelines of this project
• [x] My commit messages are signed-off
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] I have checked my code and corrected any misspellings