add `ExecutionContextVariable` support (`CURRENT_DATE`, `CURRENT_TIMESTAMP`, `CURRENT_TIMEZONE`)

[nielspardon]

[nielspardon]

Implemented by #204 (spec bump to 0.96.0). ExecutionContextVariable (CURRENT_DATE/CURRENT_TIMESTAMP/CURRENT_TIMEZONE) is fully wired:

• builder: builders/extended_expression.py execution_context_variable();
• DataFrame API: dataframe/expr.py current_timestamp / current_date / current_timezone (exported in dataframe/__init__.py);
• type inference: type_inference.py returns precision_timestamp_tz / date / string per variant.

Covered by parametrized tests in tests/dataframe/test_frame.py (all 3 pass).