feat: add option for namespace selection for telegraf-operator webhook

[SHA65536]

I added an option for the webhook to only match certain namespaces instead of all of them
In GKE they complain when you have a webhook that can intercept requests for system namespaces (kube-system, kube-node-lease) so I needed a way to adhere to their guidelines
I tested this on our company infra, seems fine
Thanks!

[SHA65536]

@skartikey @eatondustin1 can you take a look?
Sorry for the ping, saw you reviewed stuff recently

[eatondustin1]

@SHA65536 sorry for the delay, but this LGTM, thanks for the contribution 🙌

[skartikey]

@SHA65536
Issues/Suggestions:

1. Missing documentation in values.yaml - The new namespaceSelector option has no comment explaining its purpose or usage: namespaceSelector: {}
   Should include a comment like:

  # namespaceSelector allows restricting webhook to specific namespaces
  # See: https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/#matching-requests-namespaceselector
  # Example to exclude system namespaces:
  # namespaceSelector:
  #   matchExpressions:
  #   - key: kubernetes.io/metadata.name
  #     operator: NotIn
  #     values: ["kube-system", "kube-node-lease"]
  namespaceSelector: {}

2. README not updated - The chart README should document this new option for discoverability.
3. Duplicate template logic - The same webhook configuration exists in both _helpers.tpl and mutatingwebhookconfiguration.yml. Consider if this duplication is intentional or could be consolidated.
4. Template logic review - The scope: "Namespaced" is only set when namespaceSelector is not empty. This is correct behavior, but the namespaceSelector: field is always rendered (outputting namespaceSelector: {} when
   empty). This should work fine but could be wrapped in a conditional for cleaner output.

I'm dismissing my approval for now based on @skartikey 's feedback.