Add wrapped facade header for nlohmann/json

[coderabbitai]

Description

Add a wrapped facade header for nlohmann/json.hpp that includes nlohmann/json_fwd.hpp to make it clearer which header to include. This is to resolve clang-tidy warnings and simplify usage across the codebase.

Context

During the migration from submodule-based dependencies to task-based installation in the CLP project, it became unclear whether to include json.hpp or json_fwd.hpp, leading to inconsistent usage.

Links

• Referenced in PR: build(deps): Migrate core's submodule-based dependencies to task-based installation. clp#858
• Requested in comment: build(deps): Migrate core's submodule-based dependencies to task-based installation. clp#858 (comment)

Acceptance Criteria

• Create a facade header that provides a clear, single point of entry for nlohmann/json functionality
• Ensure it includes necessary forward declarations from json_fwd.hpp
• Document the usage pattern for consistent implementation across projects