perf(logs): transform bodies before constructing LogEvent

[djwhitt]

Summary

• Run copy-field, key-value enrichment, and drop-field transforms directly against the body map.
• Construct the final LogEvent once instead of allocating a new struct and {:ok, value} tuple at every stage.
• Return immediately when all transform configurations are absent or already parsed as empty.
• Specialize common binary-key path lookups while retaining get_in/2 as a fallback.
• Reuse parent maps when a requested nested drop makes no change.

Stack

1. perf(logs): reuse clean BigQuery-safe payloads #3765 — Reuse clean BigQuery-safe payloads
2. perf(logs): transform bodies before constructing LogEvent #3767 — Transform bodies before constructing LogEvent
3. perf(logs): normalize integer timestamps without digit lists #3768 — Normalize integer timestamps without digit lists
4. perf(logs): fast-path common UTC ISO 8601 timestamps #3769 — Fast-path common UTC ISO 8601 timestamps