feat: Support floating tag in product image selection

[Techassi]

Part of https://github.com/stackabletech/internal-issues/issues/169.

This PR adds support for auto updating product images. This is done by using a floating tag for product images instead of fully-qualified ones.

Significant changes are:

• Add a new useFloatingTag field to the product image selection CRD interface:

  useFloatingTag:
    default: false
    description: |-
      Use a floating tag for the product image. Defaults to `false`.
  
      This mechanism utilizes a floating image tag which always refers to the latest patch version
      in the current release line. The current release line is either automatically derived by the
      operator based on its own version, or can be overridden with `stackableVersion`.
  
      A potential newer image is only pulled when Pods are rotated or their containers are
      restarted. Pods are NOT rotated and containers are NOT restarted automatically when a new
      image is available. This behaviour makes this a passive update mechanism, rather than an
      active one.
  
      It should be noted that when this field is set to `true`, the operator uses `Always` as the
      pull policy for product images. If set to `false`, `IfNotPresent` is used. Explicitly
      setting `pullPolicy` takes precedence.
  
      ### Examples
  
      - The `stackableVersion` field is not set, the operator falls back to its own version, eg.
        26.7.0. If this field is set to `true`, the `26.7` floating tag will be used for product
        images, else, `26.7.0` will be used.
      - The `stackableVersion` field is set to `26.3.0`. If this field is set to `true`, the
        `26.3` floating tag will be used for product images, else, `26.3.0` will be used.
    type: boolean

• Remove the default of pullPolicy. This better aligns with the floating tag feature, because if enabled, the pull policy is automatically set to Always. The current default is (as far as I can see) a leftover from the past. Even the doc comment states that Always is the default, as we support floating tags (not true).

• The stackableVersion field is now a proper semantic version, instead of a plain string. This is needed, to correctly extract the floating tag if the auto update is enabled.

This PR also serves as a decision. Please vote with 👍 or 👎 or leave comments.

[sbernauer]

A few points on the CRD change:

• I like it in general, it's pretty simple to configure and understand for users.
• Naming things is hard 😓. I dislike autoUpdate because it's sounds like we are actively doing stuff and I would prefer it to be a bit more speaking. Ideas could we patchLevelUpdates or even more descritptive patchLevelFloatingTag - which makes it clear we are not actually updating anything ourselves, but just point at a floating tag. We might also to add a "real" update mechanism in the future (periodically checking for new versions and restarting), which could than get the autoUpdate (or whatever) name.
• We should clearly document that this will not automatically pull in updates, but only on Pod restarts.
  ** Also mention TLS cert rotation fixes thís implictely.

Comments on the implementations:

• For 0.0.0-dev images IMHO we want to default pullPolicy Always, as it's a giant floating tag.
  ** I had to revert a few test asserts Always -> IfNotDefault changes
• When a ":latest" image is used it's also a floating tag, so we should default to Always as well. That's what k8s seems to do as well.
  ** I also had to revert custom_without_tag and custom_with_colon_in_repo_and_without_tag test changes
• We should also warn in case the user configures a floating tag but a "wrong" pull policy.

I updated the tests and the refactored the implementation to turn them green again (one is left as an exercise for the reader). I also added some explanatory comments.
I send you the patch via Slack. Happy about comments!

[Techassi]

Naming things is hard

Agreed. Like discussed, I now went with useFloatingTag instead.

We should clearly document that this will not automatically pull in updates, but only on Pod restarts.

Yes, I think I sufficiently touched on this in my updated doc comment.

Also mention TLS cert rotation fixes thís implictely.

I would say that this is implicitly covered by my current doc comment. It basically says, that a container/pod needs to restart in order to have an effect.

All your comments on the implementation should be addressed in 83531c7.

[sbernauer]

I have to admin I prefer patchLevelFloatingTag as it makes it a bot more clear what the floatingTag is about

[sbernauer]

Any reason for the re-ordering (also in other places)?

[Techassi]

No not really, other than my OCD 🙈

I can revert if needed.

[sbernauer]

IIRC there even is a clippy lint that when constructing a struct you should use the order of the fields as it's defined in the struct.
That has product_version, stackable_version, use_floating_tag and repo, which I like :)

[sbernauer]

Bumping this up so we don't forget

I have to admin I prefer patchLevelFloatingTag as it makes it a bot more clear what the floatingTag is about