Support JSON manifest files

[JanRK]

Stolen from fluxcd/flux#2755

Describe the bug
When deciding what files to pass to kubectl, gotk ignores .json files in its target repository/branch/path.

To Reproduce
Set up a cluster with gotk. Add a valid namespace manifest file such as the one below to the target git repo/branch/path and name it *.json

{
  "apiVersion": "v1",
  "kind": "Namespace",
  "metadata": {
    "namespace": "test-namespace"
  }
}

Expected behavior
Gotk should pick up this file and apply it to the cluster under management.

What actually happens
Gotk notices that the git repo has changed, but does not apply the json file. Just renaming the json file to yaml does not work either.

Additional notes
.yaml, .yml and .json files are all accepted by kubectl, and gotk should mirror this behaviour.

[stefanprodan]

GOTK uses Kustomize for ordering, patching, labeling, etc. Kustomize only works with YAML files that's why we can't support JSON.