feat(crds): add shard field to stage spec

[krancour]

Fixes #1582

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[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 30.76%. Comparing base (8c5c1ac) to head (b2e96b8).

Files	Patch %	Lines
pkg/api/v1alpha1/types.pb.go	0.00%	4 Missing ⚠️
internal/webhook/stage/webhook.go	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1584   +/-   ##
=======================================
  Coverage   30.76%   30.76%           
=======================================
  Files         185      185           
  Lines       20091    20101   +10     
=======================================
+ Hits         6180     6184    +4     
- Misses      13628    13633    +5     
- Partials      283      284    +1     

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