Feature/optional tests

[nsoblath]

Problem statement

This PR addresses the inability to build rmqcpp without the tests and the GTest dependency. This was requested in #69.

Proposed changes

I introduced a CMake option, rmqcpp_ENABLE_TESTING to control the building of tests and finding of GTest in CMake. From the vcpkg side I moved the gtest dependency to a feature that's enabled by default. To connect the vcpkg side to the CMake side, an additional CMake variable, rmqcpp_ENABLE_TESTING_DEFAULT, controls the default value of rmqcpp_ENABLE_TESTING.

If building directly with CMake, the option can be set with -Drmqcpp_ENABLE_TESTING when cmake is called. If building with vcpkg manifest mode, these are the different options that can be used:

cmake --preset <p>                                            # -> tests ON
cmake --preset <p> -DVCPKG_MANIFEST_NO_DEFAULT_FEATURES=ON    # -> tests OFF
cmake --preset <p> -DVCPKG_MANIFEST_NO_DEFAULT_FEATURES=ON \
                   -DVCPKG_MANIFEST_FEATURES=tests            # -> tests ON 

Testing is enabled by default for backwards compatibility.

Remaining work

• Unit Tests
• Integration Tests
• Documentation