WIP: Allow retrieving heatmap from spans/individual profiles

[simonswine]

This PR implements a complete heatmap query API for visualizing profile data distribution over time and value dimensions, with support for both individual profiles and span-level data.

Key Changes:

• Heatmap Query Infrastructure: Adds new SelectHeatmap endpoint with proto definitions (HeatmapQuery, HeatmapReport, HeatmapSeries), query backend implementation for INDIVIDUAL and SPAN query types, and data aggregation pipeline with builder/merger components for processing parquet data into time/value buckets.
• Exemplar Support: Each Y-axis bucket includes optional exemplar metadata (profile ID, span ID, timestamp, labels) representing the highest-value profile in that cell, enabling drill-down from aggregated visualizations to individual profiles with little-endian span ID encoding for query consistency.
• Series Limiting & Validation: Implements TopSeries to return top-N series by total value, with input validation for step, limit, and time range parameters.