Fix instantiating Vault Secret Backend during configuration

When Secrets Backend are instantiated during configuration, not
all Airlfow packages are yet imported, because they need Secret
Backends. We have a weird cyclical relation between models,
configuration and settins which forces us to be extra careful
around configuration, settings and backends.

In this case top-level import of Connections by the Vault Secret
Backend triggered cyclic import problem (importing airflow models
require configuration to be fully loaded and initialized) but then
it could not be initialized because models needed to be imported
first.

The fix is to move Connections to be locally imported.

airflow/providers/hashicorp/secrets/vault.py

@@ -16,9 +16,8 @@
 # specific language governing permissions and limitations
 # under the License.
 """Objects relating to sourcing connections & variables from Hashicorp Vault"""
-from typing import Optional
+from typing import TYPE_CHECKING, Optional
 
-from airflow.models.connection import Connection
 from airflow.providers.hashicorp._internal_client.vault_client import _VaultClient
 from airflow.secrets import BaseSecretsBackend
 from airflow.utils.log.logging_mixin import LoggingMixin
@@ -206,7 +205,12 @@ def get_conn_uri(self, conn_id: str) -> Optional[str]:
 
         return response.get("conn_uri") if response else None
 
-    def get_connection(self, conn_id: str) -> Optional[Connection]:
+    # Make sure connection is imported this way for type checking, otherwise when importing
+    # the backend it will get a circular dependency and fail
+    if TYPE_CHECKING:
+        from airflow.models.connection import Connection
+
+    def get_connection(self, conn_id: str) -> 'Optional[Connection]':
         """
         Get connection from Vault as secret. Prioritize conn_uri if exists,
         if not fall back to normal Connection creation.
@@ -215,6 +219,10 @@ def get_connection(self, conn_id: str) -> Optional[Connection]:
         :rtype: Connection
         :return: A Connection object constructed from Vault data
         """
+        # The Connection needs to be locally imported because otherwise we get into cyclic import
+        # problems when instantiating the backend during configuration
+        from airflow.models.connection import Connection
+
         response = self.get_response(conn_id)
         if response is None:
             return None