Refactor gsPreCICE to support JSON-based configuration

[Crazy-Rich-Meghan]

Reformat and extend the gsPreCICE driver to accept simulation and coupling configuration via JSON files, using the partitioned heat conduction example as a starting point.

This allows:

• Easier runtime customization of geometry, solver settings and choose of coupling strategies.
• Clearer separation between code and configuration.

Tasks:

• Replace inline preCICE configuration (e.g., mesh name, data name) with JSON-driven logic.
• Add JSON parsing using nlohmann::json or similar. Follow the JSON schema provided by preCICE.
• Update the partitioned-heat example as the first use case, with all three types of coupling approaches.
• Provide a minimal input.json sample for the test case.