refactor(bigframes): Modularize compiler routing as proxy executor

[TrevorBergeron]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request introduces the DualCompilerProxyExecutor to manage the transition between ibis and sqlglot compilers, moving fallback logic out of BigQueryCachingExecutor. The BigQueryCachingExecutor is updated to support specific compiler selection and shared caching. Review feedback points out several critical issues in the new proxy executor, including the use of an undefined self._main attribute, missing imports for bigquery and CacheConfig, and type errors when passing labels to ExecutionSpec. Additionally, a bug was identified in bq_caching_executor.py where extra_labels were not correctly passed to the query execution method.