Guard type aliases and remove redundant imports in the postgres hook

[Dev-iL]

This attempts to fix the below issue with doc generation in the google provider observed in CI that happens while importing the postgres provider.

Traceback (most recent call last):
  File 
  "/usr/python/lib/python3.10/site-packages/sphinx/ext/autodoc/importer.py", line 143, in import_module
    return importlib.import_module(modname)
  File "/usr/python/lib/python3.10/importlib/__init__.py", line 126, in import_module
    return 
  _bootstrap._gcd_import(name, package, level)
  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1006, in 
  _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 688, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
 
  File "/opt/airflow/providers/google/src/airflow/providers/google/cloud/transfers/bigquery_to_postgres.py", line 31, in <module>
    from airflow.providers.postgres.hooks.postgres import PostgresHook
  File 
  "/opt/airflow/providers/postgres/src/airflow/providers/postgres/hooks/postgres.py", line 68, in <module>
    CursorType: TypeAlias = DictCursor | RealDictCursor | NamedTupleCursor
TypeError: unsupported operand type(s) for |: 'DictCursor' and 'RealDictCursor'

The fix is moving the two TypeAlias definitions into the existing TYPE_CHECKING block. Also, imports were cleaned up a bit.

related: #61532

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[bugraoz93]

CI looks green, thanks @Dev-iL !