Add datasource datapoint lookup by path

[sirajhoshan]

Summary

• add Datasource.get_datapoint(path) for direct datapoint lookup
• reuse the existing path filtering mechanism
• fetch at most one matching datapoint
• preserve KeyError when the requested path does not exist
• add tests for both found and missing paths

Tests

• PYTHONPATH="$PWD" python -m pytest tests/data_engine/test_datasource.py -q
• PYTHONPATH="$PWD" python -m pytest tests/data_engine/test_querying.py tests/data_engine/test_queryresult.py tests/data_engine/test_datapoint.py -q
• python -m black --check dagshub/data_engine/model/datasource.py tests/data_engine/test_datasource.py
• git diff --check

Fixes #365