Specialize char_traits for std::byte to fix from_msgpack (fixes #4756) (#4760)

* Specialize char_traits for std::byte to fix from_msgpack (fixes #4756)

Provide a char_traits<std::byte> specialization under __cpp_lib_byte
to allow parsing MessagePack data from containers of std::byte.

Signed-off-by: xuesongtap <tap91624@gmail.com>
Signed-off-by: yexiaochuan <tap91624@gmail.com>

* Fix comments for cstddef include and MessagePack tests

Signed-off-by: xuesongtap <tap91624@gmail.com>
Signed-off-by: yexiaochuan <tap91624@gmail.com>

* Fix include <cstddef> only when __cpp_lib_byte is defined and sufficient

Signed-off-by: yexiaochuan <tap91624@gmail.com>

* Fix clang-tidy warnings in MessagePack std::byte tests

Signed-off-by: yexiaochuan <tap91624@gmail.com>

* Fix handle return value in MessagePack tests

Signed-off-by: yexiaochuan <tap91624@gmail.com>

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Signed-off-by: xuesongtap <tap91624@gmail.com>
Signed-off-by: yexiaochuan <tap91624@gmail.com>