Add spec.hosts.k0sDownloadURL

[kke]

Adds a spec.hosts.[*].k0sDownloadURL field with support for token expansion:

• %% - literal %
• %p - host architecture (arm, arm64, amd64)
• %v - k0s version (v1.21.0+k0s.0)
• %x - k0s binary extension (currently always empty)

Example:

spec:
  hosts:
  - role: controller
    k0sDownloadURL: https://files.example.com/k0s%20files/k0s-%v-%p%x
    # Expands to https://files.example.com/k0s%20files/k0s-v1.21.0%2Bk0s.0-amd64

The token expansion also applies to spec.hosts.[*].files.[*].src when they're URLs.