GoogleTest should be included as ExternalProject #24

Hi!

First off, thanks for providing and maintaining this library!

I noticed that GoogleTests is checked in as a part of the source and I'd like to suggest that CMake's ExternalProject feature could be used instead. It would allow cppfs to checkout and include it in the configure process. Additionally, this could become optional based on if find_package() cannot find an already installed GTest library on the host.
Please see GoogleTest readme for more information: Incorporating Into An Existing CMake Project (https://github.com/google/googletest/blob/4f4c01d8c86659f587ea0bc27c815ffa94fc9f72/googletest/README.md)

Main benefits include

1. 3rd party code is not checked in
2. cppfs no longer need to update GoogleTest them self.
3. cppfs could easily point to specific git tag or branch of GoogleTest
4. GoogleTests source is not included if OPTION_ for tests is ON
5. If installed on host, Linking is easy as find_package(GTest WANTED_VERSION EXACT) and target_link_libraries with GTest::GTest and GTest::Main
6. If GoogleTests is downloaded and added to the project like suggested, just call target_link_libraries with gtest and gtest_main

If this sounds reasonable I would be happy to help out by sending a PR or further instructions.

Best regards