Custom BinaryType direct assignment (#5266)

* Fix discussion #4209: custom BinaryType direct assignment and extraction

When a custom BinaryType is configured (other than the default std::vector<uint8_t>),
users can now:
1. Assign values of that type directly to create binary values (not arrays)
2. Extract binary values back to that type with get<>()
3. Extract arrays to that type (for backward compatibility)

Implementation:
- Add is_compatible_binary_type trait to centralize SFINAE condition
- Update to_json to accept custom BinaryType values directly
- Update from_json to handle both binary and array inputs for custom BinaryType
- Add #include <vector> with IWYU comment to from_json.hpp
- Add comprehensive tests for assignment and array extraction
- Update binary_t documentation with example

This is purely additive and invisible to the default nlohmann::json alias, which
continues to treat std::vector<uint8_t> as arrays.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix CI: missing include and const-correctness

- Add #include <vector> to type_traits.hpp for the new
  is_compatible_binary_type trait's std::vector<std::uint8_t> reference
  (caught by cpplint's include-what-you-use check)
- Mark test-local json variables const where never reassigned
  (caught by clang-tidy's misc-const-correctness check)

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/meta/type_traits.hpp

@@ -13,6 +13,7 @@
 #include <tuple> // tuple
 #include <type_traits> // false_type, is_constructible, is_integral, is_same, true_type
 #include <utility> // declval
+#include <vector> // vector
 #if defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603L
     #include <cstddef> // byte
 #endif
@@ -632,6 +633,14 @@ template<typename BasicJsonType, typename CompatibleType>
 struct is_compatible_type
     : is_compatible_type_impl<BasicJsonType, CompatibleType> {};
 
+template<typename BasicJsonType, typename CompatibleArrayType>
+struct is_compatible_binary_type
+{
+    static constexpr bool value =
+        std::is_same<typename BasicJsonType::binary_t::container_type, CompatibleArrayType>::value &&
+        !std::is_same<typename BasicJsonType::binary_t::container_type, std::vector<std::uint8_t>>::value;
+};
+
 template<typename BasicJsonType, typename CompatibleReferenceType>
 struct is_compatible_reference_type_impl
 {