Add number of instances to describe command

[dsimansk]

Description

Currently the Status fields ActualReplicas,DesiredReplicas are set to omitempty. Therefore the current impl doesn't show any replica details if both values are 0.

➜  client git:(main) ✗ kn service describe hello                 
Name:       hello
Namespace:  default
Age:        34s
URL:        http://hello.default.127.0.0.1.nip.io

Revisions:  
  100%  @latest (hello-00001) [1] (34s)
        Image:     docker.io/dsimansk/helloworld (pinned to 29f975)
        Replicas:  1/1

Conditions:  
  OK TYPE                   AGE REASON
  ++ Ready                  20s 
  ++ ConfigurationsReady    29s 
  ++ RoutesReady            20s 

Changes

• Add number of instances (Replicas:) to kn service describe
• Add number of instances (Replicas:) to kn revision describe

Reference

Fixes #842

/assign @knative/client-reviewers

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-client-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/kn/commands/revision/describe.go	63.2%	64.4%	1.1
pkg/kn/commands/service/describe.go	84.0%	84.1%	0.1

[itsmurugappan]

/lgtm

[maximilien]

Thanks for the contribution

[maximilien]

/approve
/lgtm

[knative-prow-robot]

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