Add 'versioned-explicit-ref' annotation

[100mik]

Synthetic reference to a versioned resource.
For example:

kapp.k14s.io/versioned-explicit-ref: |
    apiVersion: v1
    kind: Secret
    name: root-password

The value of the key name is to be replaced by the latest versioned name of the referred resource
Pending:

• E2E tests

[cppforlife]

questions to be resolved:

• apigroup vs apiversion
• namespacing (especially when resource doesnt specify explicit ns). is this similar to what we do with other refs?
• cluster scoped resources (how it works today and how should ann work with that?)

[100mik]

We have to consider a resource that might wanna have multiple versioned resources referenced explicitly.

[100mik]

Changed how explicit references are structured. Multiple references are now supported using the format specified here. The format should also address our other requirements.

[100mik]

Will attempt appending existing annotation instead of adding labels. Do we also wanna avoid adding more annotations?
cc: @cppforlife

[cppforlife]

Do we also wanna avoid adding more annotations?

yup, as much as possible. (especially ones that we would have to add automatically)

[100mik]

• The annotation now has a map which maps unique keys to the latest versioned name.
• This map is populated for a resource if it's reference is found in the list of user-defined resources.

For example:
For annotation value,

{
   "references":[
      {
         "namespace":"default",
         "kind":"ConfigMap",
         "name":"config-1"
      },
      {
         "namespace":"default",
         "kind":"ConfigMap",
         "name":"config-2"
      }
   ]
}

The updated value is:

{
   "references":[
      {
         "namespace":"default",
         "apiGroup":"",
         "kind":"ConfigMap",
         "name":"config-1"
      },
      {
         "namespace":"default",
         "apiGroup":"",
         "kind":"ConfigMap",
         "name":"config-2"
      }
   ],
   "VersionedNames":{
      "default//ConfigMap/config-1":"config-1-ver-2",
      "default//ConfigMap/config-2":"config-2-ver-3"
   }
}

The versioned names are the updated ones added by kapp. Demo YAML on gist.

[cppforlife]

lets refine this a bit more...

let's support kapp.k14s.io/versioned-explicit-ref.X annotation where X can be anything like number or some string. resource can have multiple of those annotations. value of that annotation would be {"name": "...", "apiGr...}. when we match reference we would update name field with versioned name. this way this is very similar to regular k8s references:

secretRef:
  name: 
  kind: 
  ...

another question to think about is, should we use json or yaml within that annotation? e.g.

annotations:
  kapp.k14s.io/versioned-explicit-ref.X: |
    name: foo
    namespace: blah
    kind: ConfigMap