Add MajorMinor to match whole releases

[twz123]

Also introduce the VersionMatcher and VersionComparer interfaces, so that new (or custom) types can be used to match and compare versions in a canonical style.

This allows, for example, this:

// RBAC is enabled by default since Kubernetes 1.6
var rbacEnabledByDefault = version.AtLeast(version.NewMajorMinor(1, 6))

k0sVersion := version.MustParse(...)
if k0sVersion.Is(rbacEnabledByDefault) {
    ... do something related to RBAC
}

[kke]

I suppose this is more of an alternative to #21 than to #25 but at the same time it's a step towards rewriting the constraint thing as the same kind of comparators.

I like the interfaces, but they should be somehow chainable to achieve the same as the constraint syntax does, ie. something like:

comparer := version.AllowPrereleases().And(
  version.IsAtleast(version.MajorMinor(1,30)).And( // does this allow 1.30.0-rc.1? 
    version.IsOlderThanOrEqualTo(version.MustParse("1.31.4"))
  )
)


if k0sVersion.Is(comparer) {
   ...
}

vs

if version.MustConstraint(">= 1.30, <= 1.31.4").Check(k0sVersion) {
  // (does not allow any prereleases)
  ...
}

(or with #25 with prereleases allowed):

if version.MustConstraint(">>= 1.30, <<= 1.31.4").Check(k0sVersion) {
  // ( allows min 1.30.0 but not 1.30.0-alpha.1 
  //    and max 1.31.4 (including 1.31.4-alpha.1))
  ...
}

(there's now no good syntax for ~1.30 or ^1.30 or whatever that would match 1.30.0-unknownalpha.1. currently >> 1.29 gets converted to 1.29.0 and would thus match 1.29.25-alpha.1)

I believe both approaches will require runtime string parsing for some operations unless you introduce something like version.NewMinorMajorPatchPreK0s(1,30,4,"",1) to use instead of version.NewVersion("1.30.4+k0s.1") or >= 1.30.4+k0s.1

For checking if an upgrade is patch only, with this one you could do something like:

target.MajorMinor().Is(version.IsExactly(k0sVersion.MajorMinor()))
// or
target.MajorMinor().Equal(k0sVersion.MajorMinor())

instead of this in #21:

k0sVersion.IsPatchUpgrade(targetVersion)

While this MajorMinor-checking may be useful somewhere, it doesn't solve the k0sctl use case, as k0s core is effectively not versioned. Some completely incompatible k0s core change may be (and has been) introduced together with a kubernetes patch version bump or even in the same patch version but in a more recent +k0s.x buildtag.

🤷

[kke]

Newer/Older may not be the best naming as something like v1.30.1 could be older than v.1.29.20 when something is backported to the 1.29 branch