GoogleTest should be included as ExternalProject

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

Beginne mit der CMake-Konfiguration, die derzeit den eingecheckten GoogleTest-Quellcode einbindet, und vergleiche sie mit CMake's ExternalProject und GoogleTests Anleitung „Incorporating Into An Existing CMake Project“. Als erledigt gilt, wenn ein installiertes GTest mit find_package gefunden werden kann; andernfalls wird GoogleTest bei Bedarf abgerufen und die bestehenden Testziele über die entsprechenden GTest-Ziele verknüpft.

Vom Indexierungsmodell aus dem Issue-Text verfasst.