Create a common PodFlags for both Service command and ContainerSource command

[daisy-ycguo]

The specification of knative Service and eventing ContainerSource include common flags to describe the pod which will be created. I propose to create a common type under pkg/kn/commands/flags for these flags.

The specification of knative Service includes a corev1.PodSpec, while ContainerSource includes a corev1.PodTemplateSpec. corev1.PodTemplateSpec includes corev1.PodSpec. So both Service and ContainerSource commands could accept a dozen of common flags for create command and update command.

In order to provide a unique interface and to leverage the existing code base, I propose to create a common PodFlags under pkg/kn/commands/flags and move some flags in pkg/kn/commands/service/configuration_edit_flags.go there. Then this PodFlags can be used in both Service commands and ContainerSource commands.

I think below attributes in ConfigurationEditFlags are common to both Service and ContainerSource.

Image   uniqueStringArg
Env     []string
EnvFrom []string
Mount   []string
Volume  []string

Command string
Arg     []string

RequestsFlags, LimitsFlags ResourceFlags // TODO: Flag marked deprecated in release v0.15.0, remove in release v0.18.0
Resources                  knflags.ResourceOptions
Scale                      int
MinScale                   int
MaxScale                   int
ConcurrencyTarget          int
ConcurrencyLimit           int
ConcurrencyUtilization     int
AutoscaleWindow            string
Port                       string
Labels                     []string
LabelsService              []string
LabelsRevision             []string
NamePrefix                 string
<del>RevisionName               string</del>
ServiceAccountName         string
ImagePullSecrets           string
Annotations                []string
ClusterLocal               bool
User                       int64

// Preferences about how to do the action.
<del>LockToDigest         bool</del>
<del>GenerateRevisionName bool</del>
<del>ForceCreate          bool</del>

Filename string

// Bookkeeping
<del>flags []string</del>

[daisy-ycguo]

@rhuss @navidshaikh let me know your opinion to this proposed change.

[navidshaikh]

+1 for grouping the related flags together.

How about bringing PodFlags in pkg/kn/flags ?
Overall, the flag definitions are scattered in client repo: pkg/kn/flags, pkg/kn/commands/flags, pkg/kn/commads/, pkg/kn/commands/human_readable_flags.go. We can work on refactoring the flags defined in deeper pkg levels and bring them to pkg/kn/flags. This way its consistent for consumption within client and plugins.
Also, there are flags defined for properties like ResourceOptions (in pkg/kn/flags/resources.go), we could consider encapsulating them in PodFlags, this way the targeted utils like func (o *ResourceOptions) Validate() could be useful.