refactor: Unify gbq execution as semi-executor

[TrevorBergeron]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request refactors the BigFrames execution framework by replacing CacheSpec with TempTableSpec and restructuring BigQueryCachingExecutor to use a recursive _execute_bigquery method. The DirectGbqExecutor was also updated to handle the new ExecutionSpec. Several critical issues were identified in the refactored code, including missing attributes in DirectGbqExecutor (storage_manager, bqstoragereadclient, _labels, and metrics), a missing publisher argument in its constructor call, and an invalid lifetime field usage in TableOutputSpec. Furthermore, the DirectGbqExecutor currently ignores its configured compiler function, which will lead to incorrect behavior when a custom compiler is provided.

[gemini-code-assist]

DirectGbqExecutor does not have a bqstoragereadclient attribute. This will cause an AttributeError at runtime.