feat: beginning of support for multiple pipelines/freight per stage

[krancour]

Preliminary work on #1680

Note this isn't headed for main, but for a feature branch @hiddeco will be asynchronously pairing for a bit.

@hiddeco, quick brain dump of what I've gotten done so far:

I basically started with StageSpec changes -- refactoring the subscriptions into FreightSources. Then I went as far as resolving all ripples those changes created.

I have not done anything to StageStatus yet or Promotions, so Stages don't truly have the ability to host multiple pieces of Freight yet.

I have not tested this end-to-end, but I think that except for the UI, it might work.

I leave it entirely up to you if you want to merge this PR and then open your own PR to continue this work, or simply amend this PR.

Lob this whole issue back my way when you tire of it.

[codecov]

Codecov Report

Attention: Patch coverage is 77.46479% with 48 lines in your changes missing coverage. Please review.

Project coverage is 46.06%. Comparing base (cc9f958) to head (05ca41d).

Files	Patch %	Lines
internal/cli/cmd/promote/promote.go	0.00%	21 Missing ⚠️
internal/api/promote_downstream_v1alpha1.go	61.90%	7 Missing and 1 partial ⚠️
internal/controller/promotions/promotions.go	0.00%	6 Missing ⚠️
internal/kubeclient/indexer.go	50.00%	5 Missing ⚠️
internal/api/promote_to_stage_v1alpha1.go	40.00%	2 Missing and 1 partial ⚠️
internal/controller/stages/stages.go	96.10%	2 Missing and 1 partial ⚠️
internal/cli/option/flag.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##           multi-pipelines    #2154      +/-   ##
===================================================
- Coverage            46.31%   46.06%   -0.25%     
===================================================
  Files                  242      242              
  Lines                16763    16655     -108     
===================================================
- Hits                  7763     7672      -91     
+ Misses                8629     8610      -19     
- Partials               371      373       +2     

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[krancour]

@hiddeco I think this should be in a re-reviewable state.

[hiddeco]

Happy with the progress thus far and the foundation of API changes. Anything else we discover along the way can still be addressed later.

Nice work @krancour 💯