Fix breaking changes introduced with executionTier

[orbatschow]

After merging #96 there were breaking changes.
It would be great to make the executionTier optional and default it to 0.

Steps to reproduce

Deploy the following ModRule on Kubernetes 1.23:

apiVersion: api.kubemod.io/v1beta1
kind: ModRule
metadata:
  annotations:
    argocd.argoproj.io/sync-wave: "1"
  name: quay-io-container-latest-tag
  namespace: kubemod-system
spec:
  match:
  - matchFor: Any
    matchValue: Pod
    select: $.kind
  patch:
  - op: replace
    path: /spec/containers/#0/image
    select: $.spec.containers[?(@.image =~ "^(quay.io)(/)([^:]+)$")].image
    value: '{{ regexReplaceAll "^(quay.io)(/)([^:]+)$" .SelectedItem "...redacted.../${3}:latest"
      }}'
  targetNamespaceRegex: .*
  type: Patch
status: {}

With KubeMod 0.16.0, the deployment issues the following error:

Resource: "api.kubemod.io/v1beta1, Resource=modrules", GroupVersionKind: "api.kubemod.io/v1beta1, Kind=ModRule"Name: "quay-io-container-latest-tag", Namespace: "kubemod-system"for: ".": ModRule.api.kubemod.io "quay-io-container-latest-tag" is invalid: spec.executionTier: Required valueError from server (Invalid): error when applying patch:{"spec":{"match":[{"matchValue":"Pod","select":"$.kind"}]}}

With KubeMod 0.17.0, the deployment issues the following error:

error: error validating ".": error validating data: ValidationError(ModRule.spec): missing required field "executionTier" in io.kubemod.api.v1beta1.ModRule.spec; if you choose to ignore these errors, turn validation off with --validate=false