implement usage of external osi

[TimmRuppert]

Reference to a related issue in the repository

closes #9 #11

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This adds the ability to use an external OSI. E.g. in your main (parent) you can now do stuff like

...
# OSI
set(LINK_WITH_SHARED_OSI OFF)
add_subdirectory(lib/open-simulation-interface)
list(APPEND CMAKE_PREFIX_PATH ${CMAKE_BINARY_DIR}/lib/open-simulation-interface)

# asam-osi-utilities
set(USE_EXTERNAL_OSI ON)
add_subdirectory(lib/asam-osi-utilities EXCLUDE_FROM_ALL)
include_directories(${CMAKE_CURRENT_SOURCE_DIR}/lib/asam-osi-utilities/include)
...

Self-Checks

• I have added unit tests for changed or added functions.
• I have updated function documentation of changed or added functions.
• My changes generate no errors when passing CI tests.
• I have successfully implemented and tested my fix/feature locally.
• Appropriate reviewer(s) are assigned.