fix: Avoid heavy top-level imports in operators

For complex DAGs, the import time added by this library could generate
`DagBag` import timeouts if the configured value is low enough. An
Airflow documented best practice is to
[avoid heavy Python code](https://airflow.apache.org/docs/apache-airflow/stable/best-practices.html#top-level-python-code)
that runs on DAG and Operator creation, and `dbt` imports are slow,
based on profiling.

Profiling can be easily run locally, with the following command:
```shell
python -X importtime -c "from airflow_dbt_python.operators.dbt import DbtRunOperator" 2>import-times.log
```
And then parsed using a tool like
[tuna](https://github.com/nschloe/tuna).

Before this change, the operator import takes ~1.37s, which is reduced
to ~0.25s with this fix.

It's important to note that, from those 0.25s, more than 80% of the time
is spent importing `airflow` components, which will be commonly already
loaded in DAGs, so this library's import time for operators becomes
insignificant.

airflow_dbt_python/operators/dbt.py

@@ -8,15 +8,12 @@
 from dataclasses import asdict, is_dataclass
 from pathlib import Path
 from tempfile import TemporaryDirectory
-from typing import Any, Callable, Iterator, Optional, TypeVar, Union
+from typing import TYPE_CHECKING, Any, Callable, Iterator, Optional, TypeVar, Union
 
 from airflow import AirflowException
 from airflow.models.baseoperator import BaseOperator
 from airflow.models.xcom import XCOM_RETURN_KEY
 from airflow.version import version
-from dbt.contracts.results import RunExecutionResult, agate
-
-from airflow_dbt_python.hooks.dbt import BaseConfig, DbtHook, LogFormat, Output
 
 # apply_defaults is deprecated in version 2 and beyond. This allows us to
 # support version 1 and deal with the deprecation warning.
@@ -30,6 +27,12 @@ def apply_defaults(func: T) -> T:
         return func
 
 
+if TYPE_CHECKING:
+    from dbt.contracts.results import RunExecutionResult
+
+    from airflow_dbt_python.hooks.dbt import BaseConfig
+
+
 class DbtBaseOperator(BaseOperator):
     """The basic Airflow dbt operator.
 
@@ -107,6 +110,8 @@ def __init__(
         replace_on_push: bool = False,
         **kwargs,
     ) -> None:
+        from airflow_dbt_python.hooks.dbt import LogFormat
+
         super().__init__(**kwargs)
         self.project_dir = project_dir
         self.profiles_dir = profiles_dir
@@ -314,6 +319,8 @@ def prepare_directory(self, tmp_dir: str):
     def dbt_hook(self):
         """Provides an existing DbtHook or creates one."""
         if self._dbt_hook is None:
+            from airflow_dbt_python.hooks.dbt import DbtHook
+
             self._dbt_hook = DbtHook()
         return self._dbt_hook
 
@@ -611,6 +618,8 @@ def __init__(
         indirect_selection: Optional[str] = None,
         **kwargs,
     ) -> None:
+        from airflow_dbt_python.hooks.dbt import Output
+
         super().__init__(**kwargs)
         self.resource_types = resource_types
         self.select = select
@@ -757,6 +766,8 @@ def run_result_factory(data: list[tuple[Any, Any]]):
     We need to handle dt.datetime and agate.table.Table.
     The rest of the types should already be JSON-serializable.
     """
+    from dbt.contracts.results import agate
+
     d = {}
     for key, val in data:
         if isinstance(val, dt.datetime):