Add CMAKE info message before fetching content

[TimmRuppert]

Describe the feature

• The cmake directive FetchContent_MakeAvailable might take some time. It might feel like cmake hangs.
• Give the user information if a local provided OSI is used or if one is downloaded

Maybe something like this

# Locate open_simulation_interface using find_package or custom path.
if (USE_EXTERNAL_OSI)
    message(FATAL_ERROR "USE_EXTERNAL_OSI is not implemented yet")
else ()
    # Download and use our own open_simulation_interface package.
    include(FetchContent)

    # Message to indicate download is starting
    message(STATUS "Using internal OSI, starting to download open_simulation_interface...")

    FetchContent_Declare(
            open_simulation_interface
            GIT_REPOSITORY https://github.com/OpenSimulationInterface/open-simulation-interface.git
            GIT_TAG v3.7.0
    )

    # Set FETCHCONTENT_QUIET to OFF to show detailed messages about fetching.
    set(FETCHCONTENT_QUIET OFF)

    # Make the package available and display a message once done.
    FetchContent_MakeAvailable(open_simulation_interface)

    message(STATUS "Finished fetching open_simulation_interface.")

    # Set include directory variable.
    set(OSI_INCLUDE_DIR ${open_simulation_interface_BINARY_DIR})
endif ()