Add canaryReplicas field to Canary Strategy

[SimbaKingjoe]

hello rollouts community,

Problem:
Currently, to run a fixed number of canary pods (e.g., 5) independently alongside stable replicas (e.g., 20), we must use maxSurge and calculate percentages in setWeight steps. This approach is indirect and requires additional logic for configuration mapping.
Proposal:
Introduce an optional field canaryReplicas under strategy.canary.
When set (e.g., canaryReplicas: 5), the controller creates exactly that many independent canary pods. The stable replica count remains defined by spec.replicas (e.g., 20). During canary, total pod count is the sum (e.g., 25).
This provides a declarative and intuitive configuration, perfect for direct UI/API mapping without backend calculations.
Example:

spec:
  replicas: 20
  strategy:
    canary:
      canaryReplicas: 5 # 🆕 Explicit canary pod count
      steps:
        - pause: {duration: 30m} # 20 stable + 5 canary pods exist

Benefits:
Intuitive: Matches the "stable pool + canary pool" mental model.
Simple: Eliminates percentage calculations for automation.
Community feedback is welcome!

[kostis-codefresh]

You can already do this with setCanaryScale

Have you seen this property? More info https://argo-rollouts.readthedocs.io/en/stable/features/canary/#dynamic-canary-scale-with-traffic-routing