refactor: make origin a struct instead of a string

[krancour]

@hiddeco as we have previously discussed, there seems a high probability of additional kinds of Freight origins in the future (i.e. besides Warehouse).

Making this change proactively should prevent that eventuality from breaking our API again.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 52.27273% with 21 lines in your changes missing coverage. Please review.

Please upload report for BASE (multi-pipelines@62bb66f). Learn more about missing BASE report.

Files	Patch %	Lines
api/v1alpha1/stage_types.go	27.27%	7 Missing and 1 partial :warning:
internal/controller/promotions/promotions.go	0.00%	5 Missing :warning:
internal/kubeclient/indexer.go	0.00%	5 Missing :warning:
internal/controller/stages/watches.go	0.00%	2 Missing :warning:
internal/api/promote_to_stage_v1alpha1.go	0.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## multi-pipelines #2213 +/- ## ================================================== Coverage ? 45.99% ================================================== Files ? 242 Lines ? 16725 Branches ? 0 ================================================== Hits ? 7693 Misses ? 8651 Partials ? 381 ```

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