Add ONLY_SERIALIZE for NLOHMANN_DEFINE_DERIVED_TYPE_* macros

[EinarsNG]

Added additional macros to expand on the #4033 pull request.
2 new macros:

• NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE
• NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE

Works identically to the ones added in the pull request mentioned above except only allows serialization.

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• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

[coveralls]

coverage: 99.639%. remained the same
when pulling 931f466 on EinarsNG:develop
into 58f5f25 on nlohmann:develop.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks a lot!