Validate a percent encoded string and return the number of decoded bytes.

template<
    class CharSet>
result< std::size_t >
validate_pct_encoding(
    string_view s,
    CharSet const& allowed,
    pct_decode_opts const& opt = {});
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result< std::size_t >
validate_pct_encoding(
    string_view s,
    pct_decode_opts const& opt = {});
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