Use C++20 iterator concepts in our format headers

[Neverlord]

Modernize our format headers with more C++20 concepts.

[Copilot]

Pull request overview

This PR modernizes the format headers in libbroker by replacing traditional template metaprogramming patterns with C++20 concepts, improving code clarity and type safety. The changes introduce custom iterator concepts and leverage standard library concepts for more expressive template constraints.

• Introduces three new concepts in type_traits.hh: byte_output_iterator, char_output_iterator, and arithmetic
• Replaces class OutIter template parameters with concept-constrained parameters across all format headers
• Converts requires clauses and SFINAE patterns to more concise concept-based constraints (e.g., std::same_as<data>, std::integral)
• Updates iterator template parameters from generic names to standard concepts (std::input_iterator, std::sentinel_for)

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
libbroker/broker/detail/type_traits.hh	Adds forward declaration for caf::byte and defines three new concepts for output iterators and arithmetic types
libbroker/broker/format/txt.hh	Replaces unconstrained template parameters with detail::char_output_iterator, std::integral, and std::same_as concepts; adds <concepts> and <iterator> includes
libbroker/broker/format/json.hh	Applies similar concept constraints for output iterators and integral types; adds explicit include of type_traits.hh; changes iterator parameter names from ForwardIterator to Iterator with std::input_iterator concept
libbroker/broker/format/bin.hh	Replaces template constraints with detail::byte_output_iterator, std::unsigned_integral, std::same_as, and detail::arithmetic concepts; removes unused <vector> include

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