feat: add support for redis cluster mode

[Steffen911]

Important

Add support for Redis Cluster mode with configuration, validation, and tests in Helm chart.

• Behavior:
  • Adds support for Redis Cluster mode in values.yaml with redis.cluster.enabled and redis.cluster.nodes.
  • Updates README.md and charts/langfuse/README.md with configuration examples for Redis Cluster.
  • Validates Redis Cluster settings in validations.yaml to ensure proper configuration.
• Templates:
  • Updates templates/_helpers.tpl to include Redis Cluster environment variables.
  • Modifies templates/validations.yaml to add validation rules for Redis Cluster mode.
• Tests:
  • Adds redis-cluster_test.yaml with tests for Redis Cluster configuration, including standalone and cluster mode tests, override tests, validation tests, and edge cases.

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[github-actions]

Helm Unit Test Results

48 tests  +15   48 ✅ +15   2s ⏱️ ±0s
 9 suites + 1    0 💤 ± 0 
 1 files   ± 0    0 ❌ ± 0 

Results for commit 0250e90. ± Comparison against base commit 55c8955.