Added installation of helm diff (#2)

# Pull Request Description
Added installation of helm diff plugin
https://github.com/databus23/helm-diff

Solves the following problem:

[ansible-collections/kubernetes.core#174

Additionally, this problem pops up even without using Ansible on-the-fly
templating feature.

## Change type

- [X] Bug fix (non-breaking change which fixes a specific issue)
- [ ] New feature (non-breaking change adding new functionality)
- [ ] Breaking change (fix or feature that potentially causes existing
functionality to fail)
- [ ] Change that does not affect Ansible Role code (Github Actions
Workflow, Documentation, or similair)

## How was this tested?
Tested using Ansible+Vagrant on Ubuntu 20.04 and 22.04

tasks/k8s_module_prerequisites.yml

@@ -10,4 +10,12 @@
 - name: ensure k8s module dependencies are installed
   ansible.builtin.pip:
     name: "{{ longhorn_k8s_module_dependencies }}"
-    state: present
+    state: present
+
+- name: install helm diff
+  kubernetes.core.helm_plugin:
+    plugin_path: "https://github.com/databus23/helm-diff"
+    plugin_version: 3.9.5
+    state: present
+  environment:
+    KUBECONFIG: "{{ longhorn_kubeconfig_path }}"