refactor: rewrite Stage reconciler

[hiddeco]

This refactors the logic of the "regular" Stage reconciler into smaller chunks, with more extensive test coverage.

In addition, it adds conditions to the Stage to allow a better assessment of the state it is in over the previous Phase:

• Reconciling: Set for as long a reconciliation is in progress on the Stage, and e.g. new changes can be expected.
• Ready: Set to True when the Stage is in a "steady" state.
• Promoting: When True, new Freight is actively being Promoted to the Stage.
• Healthy: To indicate the health of the Stage based on the result of the health checks, compliments .status.health.
• Verified: To indicate if the Stage (Freight) has passed verification.

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 79.42318% with 371 lines in your changes missing coverage. Please review.

Project coverage is 50.86%. Comparing base (6e34eb0) to head (3e00430). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/controller/stages/regular_stages.go	75.94%	332 Missing and 23 partials :warning:
api/v1alpha1/stage_types.go	0.00%	10 Missing :warning:
internal/rollouts/analysis_template.go	97.10%	3 Missing and 1 partial :warning:
cmd/controlplane/controller.go	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2905 +/- ## ========================================== + Coverage 50.21% 50.86% +0.65% ========================================== Files 276 278 +2 Lines 24919 25113 +194 ========================================== + Hits 12512 12773 +261 + Misses 11718 11651 -67 Partials 689 689 ```

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[krancour]

One thing that tripped me up when reading through the changes, although this is not necessarily a new issue:

The method names verifyStageFreight() and verifyFreightForStage() are quite similar. The former has a good comment explaining what it does. The latter could probably stand to either be renamed or commented in a way that makes it more clear that it's about marking Freight as verified as opposed to performing verification.

[hiddeco]

The latter could probably stand to either be renamed or commented in a way that makes it more clear that it's about marking Freight as verified as opposed to performing verification.

I had this same concern, but at the time could not figure out what name would be better. "Marking" however is a good suggestion, will make the change.