Create GitHub Actions workflow for Elasticsearch cluster testing (escluster)

[cbaxley]

Part of: #737 (Eng Spec: Add clustering support for Elasticsearch)

Description
As a maintainer, I want automated CI/CD testing for Elasticsearch cluster deployments so that cluster functionality regressions are caught before merging.

Acceptance criteria

• Workflow triggers on PRs to develop and main branches
• Workflow provisions multi-node infrastructure for cluster testing
• Workflow installs LME master node with cluster mode enabled
• Workflow runs elasticsearch.yml to deploy cluster nodes
• Workflow verifies all nodes join the cluster successfully
• Workflow verifies cluster health is green
• Workflow runs API tests against the cluster
• Workflow cleans up resources on completion and failure