### Description

Support mounting to multiple namespaces with the Hashicorp Vault Secrets Back-end

### Use case/motivation

As a data engineer I wish to utilize secrets stored in multiple mount paths (to support connecting to multiple namespaces)  without having to mount to a higher up namespace.

### Related issues

_No response_

### Are you willing to submit a PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

"os.environ",
        {
            "AIRFLOW_VAR_HELLO": "world",
        },
    )
    @mock.patch("airflow.providers.hashicorp._internal_client.vault_client.hvac")
    def test_get_variable_value_non_existent_key(self, mock_hvac):
        """
        Test that if the key with connection ID is not present in Vault, _VaultClient.get_connection
        should return None
        """
        mock_client = mock.MagicMock()
        mock_hvac.Client.return_value = mock_client
        
        mock_client.secrets.kv.v2.read_secret_version.side_effect = InvalidPath()
        kwargs = {
            "variables_path": "variables",
            "mount_point": "airflow",
            "auth_type": "token",
            "url": "http://127.0.0.1:8200",
            "token": "s.7AU0I51yv1Q1lxOIg1F3ZRAS",
        }
        test_client = VaultBackend(**kwargs)
        assert test_client.get_variable("hello") is None
        mock_client.secrets.kv.v2.read_secret_version.assert_called_once_with(
            mount_point="airflow", path="variables/hello", version=None
        )
        assert test_client.get_variable("hello") is None
    @mock.patch("airflow.providers.hashicorp._internal_client.vault_client.hvac")
    def test_auth_failure_raises_error(self, mock_hvac):