feat: add support for custom header sanitization and redaction

[AdamMagued]

Overview

Currently, httpretty automatically sanitizes default sensitive headers (Authorization, Proxy-Authorization, Cookie, and Set-Cookie) using fixed block characters, but callers cannot configure custom headers for redaction or provide custom sanitization routines without completely bypassing sanitization via SkipSanitize. Because the underlying internal/header package is internal, external callers also cannot reuse standard sanitizers.

This change introduces configurable header sanitization and redaction options on Logger.

Details

• Added SanitizeHeader(headers []string) and RedactHeaders(headers []string) methods to Logger, as well as SanitizeHeaders []string on the struct, allowing callers to specify custom headers (such as X-Api-Key or custom auth tokens) to redact with [REDACTED]. If Authorization or Cookie is specified in this list, it is redacted with [REDACTED] instead of the default block characters.
• Added SetHeaderSanitizer(f HeaderSanitizer) method and HeaderSanitizer HeaderSanitizer field to Logger. Custom sanitizers receive (name, value string) and return the sanitized string. Panics within user-provided sanitizers are safely recovered and reported without crashing. If the custom sanitizer returns the original value, fallback to default sanitizers is preserved.
• Exported AuthorizationSanitizer, CookieSanitizer, and SetCookieSanitizer from package httpretty wrapping internal/header, allowing callers to reuse built-in sanitizers in custom sanitization rules.
• Added regression tests in logger_test.go verifying header list redaction, custom sanitizer functions, panic recovery, case insensitivity, response header sanitization, concurrency safety, and SkipSanitize behavior.

Fixes #14