Add support for version aliases

[dgn]

In the past, we only supported selecting a minor version in spec.version, like v1.23. Today, we only support selecting a specific patch version. We should add an option to use the old way of selecting a minor version. When coupled with fully-automatic proxy updates, this could be useful for customers that want to make sure they always have the latest security fixes in both proxies and control plane.

The version aliases will be defined explicitly in versions.yaml:

versions:
  - name: latest-unstable
    ref: v1.25-alpha.d547b8580cf6298e15ba732823b2e027071516ea
  - name: v1.24-latest
    ref: v1.24.2
  - name: v1.24.2
    version: 1.24.2
    repo: https://github.com/istio/istio
    commit: 1.24.2
    charts:
      - https://istio-release.storage.googleapis.com/charts/base-1.24.2.tgz
      - https://istio-release.storage.googleapis.com/charts/istiod-1.24.2.tgz
      - https://istio-release.storage.googleapis.com/charts/gateway-1.24.2.tgz
      - https://istio-release.storage.googleapis.com/charts/cni-1.24.2.tgz
      - https://istio-release.storage.googleapis.com/charts/ztunnel-1.24.2.tgz
[...]

When a user sets an Istio's spec.version field to an alias, the operator will resolve it and create an IstioRevision with the underlying actual version:

kind: Istio
spec:
  version: v1.24-latest
---
kind: IstioRevision
spec:
  version: 1.24.4

• Update scripts to add aliases
• Update versions.yaml parser to understand aliases
• Make versions.yaml available to the controllers
• Add tests to verify the minor version aliases point to the latest patch versions
• Add tests for automatic patch updates
• Make sure existing tests also test version aliases
• Document the new version aliases and their behavior