Feature: Add optional 'leases' etcd shard

[apeabody]

Adds support for configuring an optional leases etcd shard. This shard is specifically designed for megaclusters to isolate the massive volume of high-frequency write traffic generated by node heartbeats. By redirecting this traffic to a dedicated leases shard, it prevents the primary etcd database from being overwhelmed, ensuring overall control plane stability.

• Allows 'leases' API validation
• Maps well-known port assignments
• Configures API Server overrides and nodeup DNS mapping
• Updates Etcd Manager and TLS Configurations
• Implements Node diagnostics, E2E integrations, and log overrides
• Introduces mock test fixtures in cluster.go

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[hakman]

/ok-to-test

[hakman]

@apeabody Could you add some info in the description on why this is useful and who should use it?

[apeabody]

@apeabody Could you add some info in the description on why this is useful and who should use it?

Thanks @hakman - Also added to the create cluster documentation.

[rifelpet]

Is there value in giving this etcd cluster persistent remote storage like EBS volumes? @hakman I'm wondering if we could just give it a hostPath volume mount instead.