feat(shield): add support to cert-manager on cluster-shield

[francesco-furlan]

What this PR does / why we need it:

Add cert-manager support to generate the tls certs for audit/admission-control features.

cluster:
  tls_certificates:
    cert_manager:
      # Enable cert-manager for certificate management
      enabled: false
      ca:
        # Create the CA certificate using cert-manager
        create: false
        # The template for the CA certificate secret (if generate is true)
        # will automatically add the annotation `cert-manager.io/allow-direct-injection: "true"` if not present
        secret_template: {}
        # The name of the existing CA certificate secret (if generate is false)
        # has to be annotated with `cert-manager.io/allow-direct-injection: "true"`
        secret_name: ""
        # The namespace of the existing CA certificate secret (if generate is false)
        secret_namespace: ""
      issuer:
        # Create the Issuer instead of using an existing one
        create: false
        # The name of the existing issuer
        name: ""
        # The kind of the existing issuer (Issuer, ClusterIssuer)
        kind: Issuer
        # The group of the existing issuer
        group: cert-manager.io
      # Certificate duration (default: 30 days)
      duration: "720h"
      # How long before expiry to renew (default: 15 days)
      renew_before: "360h"

Thanks @yoderme for the proposal of this feature, and the initial setup in this PR

Checklist

• [x] Title of the PR starts with type and scope, (e.g. feat(agent,node-analyzer,sysdig-deploy):)
• [x] Chart Version bumped for the respective charts
• [x] Variables are documented in the README.md (or README.tpl in some charts)
• [x] Check GithubAction checks (like lint) to avoid merge-check stoppers
• [x] All test files are added in the tests folder of their respective chart and have a "_test" suffix