minor refactoring of WarehouseSpec

[krancour]

Fixes #954

Before:

apiVersion: kargo.akuity.io/v1alpha1
kind: Warehouse
metadata:
  name: my-warehouse
  namespace: kargo-demo
spec:
  subscriptions:
    images:
    - repoURL: krancour/nginx
      semverConstraint: ^1.24.0

After:

apiVersion: kargo.akuity.io/v1alpha1
kind: Warehouse
metadata:
  name: my-warehouse
  namespace: kargo-demo
spec:
  subscriptions:
  - image:
      repoURL: krancour/nginx
      semverConstraint: ^1.24.0

[netlify]

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Name	Link
🔨 Latest commit	8ec263b
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[codecov]

Codecov Report

Attention: 26 lines in your changes are missing coverage. Please review.

Comparison is base (8066ac0) 50.77% compared to head (8ec263b) 50.85%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #986      +/-   ##
==========================================
+ Coverage   50.77%   50.85%   +0.07%     
==========================================
  Files         104      104              
  Lines        7573     7585      +12     
==========================================
+ Hits         3845     3857      +12     
+ Misses       3581     3580       -1     
- Partials      147      148       +1     

Files	Coverage Δ
api/v1alpha1/warehouse_types.go	0.00% <ø> (ø)
internal/controller/warehouses/warehouses.go	63.47% <100.00%> (+4.17%)	⬆️
internal/controller/warehouses/git.go	72.83% <50.00%> (-1.52%)	⬇️
internal/controller/warehouses/helm.go	85.93% <60.00%> (-2.40%)	⬇️
internal/controller/warehouses/images.go	73.33% <50.00%> (-2.11%)	⬇️
internal/webhook/warehouse/webhook.go	91.81% <88.46%> (-2.36%)	⬇️
api/v1alpha1/zz_generated.deepcopy.go	10.10% <0.00%> (ø)

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[gdsoumya]

LGTM