Add implementation for kapp-controller GetAvailablePackages.

[absoludity]

Description of the change

Adds the implementation for the kapp_controller plugin's GetAvailablePackages endpoint.

While port-forwarding to the kubeapps-apis service with

kubectl -n kubeapps port-forward svc/kubeapps-internal-kubeappsapis 8080:8080

http endpoint responds with:

curl -s http://localhost:8080/kapp_controller/packages/v1alpha1 | jq .
{
  "packages": [
    {
      "name": "app1.example.com",
      "version": "0.11.3",
      "iconUrl": "",
      "repository": null
    },
    {
      "name": "app2.example.com",
      "version": "3.11.3",
      "iconUrl": "",
      "repository": null
    },

while with grpcurl:

grpcurl -plaintext localhost:8080 kubeappsapis.plugins.kapp_controller.packages.v1alpha1.PackagesService.GetAvailablePackages
{
  "packages": [
    {
      "name": "app1.example.com",
      "version": "0.11.3"
    },
    {
      "name": "app2.example.com",
      "version": "3.11.3"
    },
    ...

Note: I still need to check the fake dynamic client to see the form unit-tests will take. I'll those in a subsequent PR.

Benefits

Demonstrate using the dynamic client to query the plugin backend.

Applicable issues

• Ref Implement kapp-controller/flux Kubeapps APIs for GetAvailablePackages #2775

[absoludity]

Rubber-stamping this PR since it affects only the experimental apis service and the following PR (#2840) is ready to land and will unblock others from being able to work on this too.

[antgamdia]

Great! I didn't manage to have a look at this on time :S (sorry!)
I've installed Carvel tools, installed a Package and a PackageRepository and the API works perfectly. Thank you!