Optional dependency to gtest

In vcpkg package manager rmqcpp has dependency to gtest.
https://github.com/microsoft/vcpkg/blob/master/ports/rmqcpp/vcpkg.json
Normally, gtest package is unnecessary for using the library and could be included optionally.

Start with the rmqcpp vcpkg manifest at the linked microsoft/vcpkg ports/rmqcpp/vcpkg.json entry. Check how gtest is declared and how vcpkg represents optional or test-only dependencies. Done means normal rmqcpp installation no longer requires gtest, while the project's tests can still use it if needed.

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