[WIP] Refactor CMakeLists.txt to support default shared library builds

[Copilot]

• Analyzed the current CMakeLists.txt in power_grid_model_c/power_grid_model_c/ directory
• Confirmed that BUILD_SHARED_LIBS is not currently used anywhere in the repository
• Identified the target file that needs modification: power_grid_model_c/power_grid_model_c/CMakeLists.txt
• Found the line that needs changing: add_library(power_grid_model_c SHARED on line 6
• Implement the DEFAULT_SHARED variable logic to control library type
• Test the implementation with both BUILD_SHARED_LIBS defined and undefined scenarios
• Verify the changes work for both standalone and submodule usage patterns
• Validate that no existing functionality is broken

Summary: Need to refactor the CMakeLists.txt to use a DEFAULT_SHARED variable that checks if BUILD_SHARED_LIBS is defined by parent/user, defaulting to SHARED if not set. The current hardcoded SHARED specification needs to be made conditional while maintaining compatibility.

Original prompt

Refactor the CMakeLists.txt in the power_grid_model_c directory to support default shared library builds unless BUILD_SHARED_LIBS is set by the parent or user. Use a DEFAULT_SHARED variable to concisely control the add_library type, avoiding code duplication. This ensures maximum compatibility for both standalone and submodule usage, while minimizing changes to the repository's structure or logic. The implementation should check if BUILD_SHARED_LIBS is defined; if so, add_library uses the default (user/parent controlled); otherwise, add_library uses SHARED for the power_grid_model_c target. Do not set or overwrite BUILD_SHARED_LIBS anywhere in the repo. Apply only the necessary minimal changes for this logic.

This pull request was created as a result of the following prompt from Copilot chat.

Refactor the CMakeLists.txt in the power_grid_model_c directory to support default shared library builds unless BUILD_SHARED_LIBS is set by the parent or user. Use a DEFAULT_SHARED variable to concisely control the add_library type, avoiding code duplication. This ensures maximum compatibility for both standalone and submodule usage, while minimizing changes to the repository's structure or logic. The implementation should check if BUILD_SHARED_LIBS is defined; if so, add_library uses the default (user/parent controlled); otherwise, add_library uses SHARED for the power_grid_model_c target. Do not set or overwrite BUILD_SHARED_LIBS anywhere in the repo. Apply only the necessary minimal changes for this logic.

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