Implement ECK monitoring support for presenting client certificates to elasticsearch

[pkoutsovasilis]

Summary

This PR adds client certificate support to the stack monitoring (Metricbeat/Filebeat sidecar) infrastructure so that monitoring sidecars can authenticate via mTLS when connecting to Elasticsearch clusters that have client authentication enabled. This covers two distinct scenarios: the Metricbeat sidecar connecting to the source (monitored) Elasticsearch over its HTTP endpoint, and the monitoring output connecting to the target (monitoring) Elasticsearch cluster.

Relates #9081

Changes

• Common stackmon output config: Extended buildOutputConfig to return a third value — a client certificate volume — when the monitoring target ES association has clientCertSecretName configured. Adds ssl.certificate and ssl.key to the Beat output configuration pointing to the mounted client certificate files
• Common stackmon sidecar: Extended NewMetricBeatSidecar to accept an optional clientCertVolume parameter, which is mounted into the sidecar container alongside the existing CA volume
• Elasticsearch Metricbeat sidecar: When the monitored Elasticsearch cluster has client authentication enabled (clientAuthenticationRequired), the operator mounts the internal operator client certificate into the Metricbeat sidecar so it can authenticate to the local ES HTTP endpoint via mTLS. The clientAuthenticationRequired
  flag is threaded through WithMonitoring, ReconcileConfigSecrets, and Metricbeat
• E2e tests: Added TestESStackClientAuthTransitionMonitored (verifies monitoring continues working when the monitored ES transitions from client auth enabled to disabled) and TestESStackClientAuthTransitionMonitoring (verifies monitoring continues working when the monitoring target ES transitions from client auth enabled to
  disabled)

[prodsecmachine]

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[pkoutsovasilis]

buildkite test this -f p=gke,t=TestESStackClientAuthTransition.*,E2E_TAGS=es -m s=9.3.3,s=8.19.14,s=9.2.8

[pkoutsovasilis]

buildkite test this -f p=gke,t=TestESStackClientAuthTransition.*,E2E_TAGS=es -m s=9.3.3,s=8.19.14,s=9.2.8,s=9.4.0-SNAPSHOT