Implement GET /api/v1/features endpoint #7467

Summary

Implement the GET /api/v1/features endpoint to expose which features and capabilities are enabled in a Cortex instance.

Background

Prometheus introduced this endpoint to allow clients (e.g., Grafana) to discover which features a backend supports without relying on version strings from /status/buildinfo. This is especially important for Prometheus API implementations like Cortex, where version numbers don't map to Prometheus feature availability.

See the original Prometheus proposal: prometheus/prometheus#10022

Motivation

• Grafana and other clients use this endpoint to enable/disable UI features (e.g., exemplars, label matchers on label-values requests)
• Cortex has several feature-gated capabilities (streaming ingestion, query sharding, parquet queryable, remote write v2, etc.) with no single endpoint to discover what's active
• Operators currently have to parse /config output or logs to determine which experimental features are enabled

Scope

• Register GET /api/v1/features on both the Prometheus and legacy HTTP prefixes
• Return a response compatible with the Prometheus format
• Include Cortex-specific feature flags and experimental capabilities