feat: introduce readyReplicas for Vertex and MonoVertex

[whynowy]

1. Enabled meaningful readiness check for Vertex and MonoVertex pods, to tell if the pod is ready to take traffic.
2. Introduced readyReplicas in the status fields, that will be used to do more accurate autoscaling - do not want to do it in this PR even though it's a small change, let the CRD change out first.

Enhancement for readiness check on each sidecar: #2015.

[codecov]

Codecov Report

Attention: Patch coverage is 59.37500% with 13 lines in your changes missing coverage. Please review.

Project coverage is 61.31%. Comparing base (ee8b83a) to head (6e3a51a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/metrics/metrics_server.go	0.00%	6 Missing ⚠️
pkg/reconciler/monovertex/controller.go	0.00%	5 Missing ⚠️
pkg/reconciler/vertex/controller.go	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2014      +/-   ##
==========================================
+ Coverage   61.07%   61.31%   +0.23%     
==========================================
  Files         317      317              
  Lines       28375    28396      +21     
==========================================
+ Hits        17331    17411      +80     
+ Misses      10098    10038      -60     
- Partials      946      947       +1     

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