[Feature]: Optionally support mutual TLS between Kibana and Elasticsearch

[afeefghannam89]

Kibana currently authenticates to Elasticsearch with username/password over standard TLS
(the Elastic default). The role already generates a client certificate (<host>-kibana.p12)
via elasticsearch-certutil and copies it to /etc/kibana/certs, but it is never
referenced in kibana.yml — so it is unused today.

Add opt-in mutual TLS (default: disabled):

• New toggle (e.g. kibana_elasticsearch_mtls, default false).
• When enabled, Kibana presents a client certificate. Support two sources:
  • Collection-generated (default): use the already-generated <host>-kibana.p12
    (signed by the shared CA). Gives today's unused certificate a purpose and fits the
    standard setup where Elasticsearch trusts the shared CA.
  • Bring your own: let the user point to their own client certificate/key
    (e.g. kibana_elasticsearch_cert / kibana_elasticsearch_key, or a keystore). Needed
    when Elasticsearch trusts a corporate/external CA instead of the collection's — the
    client cert must then be signed by that CA, so the generated one would be rejected.
  • Wire the chosen source into kibana.yml
    (elasticsearch.ssl.certificate/key or elasticsearch.ssl.keystore.path).
• Document that Elasticsearch must request client certs
  (xpack.security.http.ssl.client_authentication: optional, typically with a PKI realm —
  Elastic recommends optional, not required).
• When disabled (default): do not generate the client certificate — removing today's
  unused kibana.p12.

Default behaviour stays credential-based, so this is additive and non-breaking. The
bring-your-own path pairs with the own-CA trust from #483 (Kibana verifying Elasticsearch)
for a full corporate-PKI setup. Depends on #483.