refactor: use String& instead of std::string_view in string_util

[kajukitli]

Follow-up to #1904

Uses the protocol's String type alias (std::string) directly instead of std::string_view for the CharactersUTF8, CharacterCount, CharactersLatin1, and CharactersUTF16 functions.

This is more consistent with the rest of the codebase and avoids the C++ <string_view> header dependency.

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