Add leakage parameter for government infrastructure investment

[MahadevBalla]

Closes #1086

Summary

• Add new parameter infra_investment_leakage_rate to default_parameters.json
• Apply leakage to public capital accumulation in fiscal.get_K_g()
• Update the get_K_g() docstring to reflect the new law of motion
• Add tests for the no-leakage baseline and a positive-leakage SS case

Testing

• pytest tests/test_fiscal.py
• pytest tests/test_fiscal.py -k "get_K_g"

Output / Validation

pytest tests/test_fiscal.py -k "get_K_g"
3 passed, 23 deselected in 4.38s

pytest tests/test_fiscal.py
26 passed in 8.77s