Import Connection lazily in hooks to avoid cycles (#15361)

The current implementation imports Connection on import time, which
causes a circular import when a model class needs to reference a hook
class.

By applying this fix, the airflow.hooks package is completely decoupled
with airflow.models on import time, so model code can reference hooks.
Hooks, on the other hand, generally don't reference model classes.

Fix #15325.

(cherry picked from commit 7560316084)

airflow/hooks/base.py

@@ -18,12 +18,14 @@
 """Base class for all hooks"""
 import logging
 import warnings
-from typing import Any, Dict, List
+from typing import TYPE_CHECKING, Any, Dict, List
 
-from airflow.models.connection import Connection
 from airflow.typing_compat import Protocol
 from airflow.utils.log.logging_mixin import LoggingMixin
 
+if TYPE_CHECKING:
+    from airflow.models.connection import Connection  # Avoid circular imports.
+
 log = logging.getLogger(__name__)
 
 
@@ -37,7 +39,7 @@ class BaseHook(LoggingMixin):
     """
 
     @classmethod
-    def get_connections(cls, conn_id: str) -> List[Connection]:
+    def get_connections(cls, conn_id: str) -> List["Connection"]:
         """
         Get all connections as an iterable, given the connection id.
 
@@ -53,13 +55,15 @@ class BaseHook(LoggingMixin):
         return [cls.get_connection(conn_id)]
 
     @classmethod
-    def get_connection(cls, conn_id: str) -> Connection:
+    def get_connection(cls, conn_id: str) -> "Connection":
         """
         Get connection, given connection id.
 
         :param conn_id: connection id
         :return: connection
         """
+        from airflow.models.connection import Connection
+
         conn = Connection.get_connection_from_secrets(conn_id)
         if conn.host:
             log.info(