PERF: Speed up DatetimeIndex field accessors with ZoneInfo timezones

[shubhamgoel27]

Summary

• Routes ZoneInfo timezones through the fast C-level binary search path for UTC-to-local conversion, instead of the slow per-timestamp Python tz.utcoffset() API path
• Reuses dateutil's existing transition data extraction via dateutil_gettz(tz.key) — no custom TZif parsing needed
• Fixes tz_cache_key() to return tz.key for ZoneInfo, enabling DST transition data caching

Root cause

In Localizer.__cinit__ (tzconversion.pyx), ZoneInfo was grouped with tzlocal:

elif is_tzlocal(tz) or is_zoneinfo(tz):
    self.use_tzlocal = True

This forced every timestamp through _tz_localize_using_tzinfo_api(), which makes a Python-level tz.utcoffset() call per element — O(n) Python API calls instead of a single O(n) C-level loop with O(log k) binary search on cached transition arrays.

Fix

1. tz_cache_key(): Return tz.key for ZoneInfo (was returning None, preventing caching)
2. get_dst_info(): Add ZoneInfo branch that converts to the equivalent dateutil timezone via dateutil_gettz(tz.key) and reuses dateutil's transition data extraction (dateutil reads the same underlying TZif files)
3. Localizer.__cinit__: Remove is_zoneinfo(tz) from use_tzlocal check so ZoneInfo flows through get_dst_info → binary search path

Benchmark

DatetimeIndex.month with 1M elements, tz=ZoneInfo('US/Eastern'):
  Before: ~3,000ms  (per-timestamp Python API calls)
  After:     ~29ms  (C-level binary search on cached transition arrays)
  Speedup: ~100x

Correctness verified against both dateutil and pytz across multiple timezones including DST spring-forward and fall-back transitions.

Closes #64363

Test plan

• All existing timezone tests pass (6,574 tests across tslibs, datetime indexes, timestamps, extensions, frame/series tz methods)
• Verified correctness for DST-aware (US/Eastern, Europe/London, Australia/Sydney), non-DST (Asia/Tokyo), and fixed-offset (Etc/GMT+5) ZoneInfo timezones
• Verified DST transition edge cases (spring forward, fall back)
• Results match both dateutil and pytz for .year, .month, .day, .hour accessors

🤖 Generated with Claude Code

[jbrockmendel]

I suspect this would also close #58962