Add version constraints

[kke]

As discussed in k0sproject/k0sctl#547

Implementing a k0s feature gate / registry will require a way to validate versions against constraints.

This PR adds a convenient version constraint system.

c := version.MustConstraint(">= v1.24, < 1.26.0")
println(c.CheckString("v1.24.0+k0s.1"))
// true

Which is a lot more convenient than something like:

targetVmin, err := version.NewVersion("v1.24.0")
if err != nil {
  return err
}
targetVmax, err := version.NewVersion("v1.26.0")
if err != nil {
  return err
}

k0sVersion, err := version.NewVersion(p.Config.Spec.K0s.Version)
if err != nil {
  return err
}

if k0sVersion.GreaterThanOrEqual(targetVmin) && k0sVersion.LessThan(targetVmax) {
  // do stuff
} else if .......

The ruby style "pessimistic" ~> 1.0 (roughly the same as >= 1.0.0, < 2.0.0) operator was not implemented.

The k0s versioning scheme doc does not address pre-release versions, but something like v1.24.0-alpha.1+k0s.0 should act as expected. Not entirely sure about v1.24.0+k0s.1-rc.1 or v1.24.0-alpha.1+k0s.1-rc.1 in case that is possible.