Resource extensions implementation

[pPrecel]

Description

Changes proposed in this pull request:

• fetch extensions from cluster at the end of every cli run

Related issue(s)

• Prototype extending CLI command group solution #2228

[pPrecel]

Tests results

I tested new feature in two scenarios (with/without cluster connection) and compare it to the built cli without this fueature. Keep in mind, that results are inaccurate and not deterministic, but show the trend and the scale.

Before adding features

time ./kyma-cli alpha
________________________________________________________
Executed in   27.30 millis    fish           external
   usr time   11.74 millis   40.00 micros   11.70 millis
   sys time    8.02 millis  703.00 micros    7.32 millis

NOTE: 27 millis is equal to 0.027 seconds

New cli with cluster connection

export KUBECONFIG=<PATH_TO_KUBECONFIG>
time ./kyma-cli-new alpha
________________________________________________________
Executed in  730.25 millis    fish           external
   usr time   19.16 millis    0.09 millis   19.07 millis
   sys time   27.89 millis    1.09 millis   26.80 millis

NOTE: 730 millis is equal to 0.73 seconds

New without cluster

export KUBECONFIG="" # reset to default - no cluster
time ./kyma-cli-new alpha
________________________________________________________
Executed in   24.27 millis    fish           external
   usr time   11.82 millis   37.00 micros   11.79 millis
   sys time    8.83 millis  625.00 micros    8.21 millis

NOTE: 24 millis is equal to 0.024 seconds

Data

I've tested it with self-crafted example of the extension for the Function resources:

ConfigMap:

apiVersion: v1
kind: ConfigMap
metadata:
  labels:
    kyma-cli/extension: resource
  name: function.extension.cli
data:
  rootCommand: |
    name: function
    description: manage functions
    descriptionLong: use this command to manage function resources on a cluster
  resource: |
    scope: namespace
    kind: Function
    group: serverless.kyma-project.io
    version: v1alpha2
    singular: function
    plural: functions
  templateCommands: |
    explain:
      description: explains Kyma Functions
      descriptionLong: use this command to explain Kyma Serverless Function CR goal
      output: |-
        Kyma Serverless Module offers a Kubernetes-based platform on which you can build,
        run, and manage serverless applications in Kubernetes. These applications are called Functions,
        and they are based on the Function custom resource (CR) objects.
        They contain simple code snippets that implement specific business logic.
        For example, you can define that you want to use a Function as a proxy that saves all incoming
        event details to an external database.

        Such a Function can be:

        - Triggered by other workloads in the cluster (in-cluster events) or business events coming from
          external sources. You can subscribe to them using a Subscription CR.
        - Exposed to an external endpoint (HTTPS). With an APIRule CR, you can define who can reach the endpoint
          and what operations they can perform on it.
        
        Read more about serverless Functions here:
        https://kyma-project.io/#/serverless-manager/user/README?id=what-is-serverless

After applying this ConfigMap it should be possible to run commands like these:

go run main.go alpha # <- here we should see new command called "function"
go run main.go alpha function # <- this should run help of new function and print another new command "explain"
go run main.go alpha function explain # <- this should print text from ConfigMap from path .data.genericCommands.explain.explainText