fix: clean up Teradata connection parameters

[miachillgood]

Summary

• let the Teradata driver supply its own port and authentication defaults when those options are omitted
• apply explicit connection options after json-params, so command-line values take precedence
• document the precedence and driver parameter names, with regression tests for both behaviors

Validation

• python -m pytest tests/unit/ibis_teradata/test_init.py (6 passed)
• python -m black --check third_party/ibis/ibis_teradata/api.py third_party/ibis/ibis_teradata/__init__.py tests/unit/ibis_teradata/test_init.py
• python -m flake8 tests/unit/ibis_teradata/test_init.py
• git diff --check

Fixes #1016

[nj1973]

/gcbrun

[nj1973]

These changes look good, thanks for contributing to DVT!