zapcore: Add PreWriteHook for transforming entries before write

[sywhang]

This adds a new CheckPreWriteHook type and a Before method on CheckedEntry that allows transforming the Entry and Fields before they are written to any registered Cores.

The existing CheckWriteHook (set via After/WithFatalHook) only runs after cores have already written the entry, which means it cannot modify what gets logged. PreWriteHook fills that gap: it runs before any Core.Write call, allowing callers to enrich, redact, or reshape log entries and fields in a composable way.

Multiple pre-write hooks run in the order they were added. Like other CheckedEntry methods, Before is safe to call on nil references.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.39%. Comparing base (16fb16b) to head (aef708b).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
zapcore/entry.go	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1534      +/-   ##
==========================================
- Coverage   98.54%   98.39%   -0.16%     
==========================================
  Files          53       53              
  Lines        3033     3052      +19     
==========================================
+ Hits         2989     3003      +14     
- Misses         35       41       +6     
+ Partials        9        8       -1     

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