[unordered-dense] New port

[RT2Code]

Add the unordered-dense port : https://github.com/martinus/unordered_dense

• Changes comply with the maintainer guide
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• Optional dependencies are resolved in exactly one way. For example, if the component is built with CMake, all find_package calls are REQUIRED, are satisfied by vcpkg.json's declared dependencies, or disabled with CMAKE_DISABLE_FIND_PACKAGE_Xxx
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• The license declaration in vcpkg.json matches what upstream says.
• The installed as the "copyright" file matches what upstream says.
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• The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
• Only one version is in the new port's versions file.
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