Allow to use an existing Catch2 instead of forcing the use of FetchContent

See https://github.com/gbionics/lie-group-controllers/blob/31cedaa1bbb7f31a6fc65aff52f4c97c54b1e96b/cmake/AddLieGroupControllersUnitTest.cmake#L5 . Ideally we should increase the catch2 version used to use catch2 3.*, and add `FIND_PACKAGE_ARGS` to the `FetchContent_MakeAvailable` call, and in general use a modern way of using FetchContent.

Start with CMakeLists and AddLieGroupControllersUnitTest.cmake at line 5, then review the FetchContent_MakeAvailable call and the Catch2 version it requests. Check modern FetchContent usage and how FIND_PACKAGE_ARGS can permit an existing Catch2 3.* installation. Done means the project can use an installed Catch2 instead of always fetching it, while retaining the unit-test setup.

Written by the indexing model from the issue text.