Refactor PyUtf8Str - #6047

cc @ever0de

I am going to add a new validation step for PyUtf8Str. This is useful for sqlite3

Summary by CodeRabbit

• New Features

  • Introduced a distinct UTF-8 string type for Python strings, enabling stricter UTF-8 validation and handling.
  • Added new methods for safe downcasting and UTF-8 specific string representations in object and protocol handling.
  • Expanded public interfaces to include UTF-8 string types.
  • Added a macro for retrieving UTF-8 validated identifiers.

• Bug Fixes

  • Improved error handling for string formatting and output involving invalid UTF-8 or surrogate characters.
  • Enhanced string comparison and attribute handling to support both UTF-8 and WTF-8 encoded strings.

• Refactor

  • Updated numerous function signatures and internal logic to require or return UTF-8 validated string references, especially in I/O, database, and standard library modules.
  • Simplified and standardized string conversions and representations throughout the system.
  • Refined downcasting mechanisms with VM-based validation for safer type conversions.

• Style

  • Streamlined code for string formatting, representation, and output for consistency and maintainability.