### 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)

assert "world" == returned_uri
    @mock.patch("airflow.providers.hashicorp._internal_client.vault_client.hvac")
    def test_get_variable_value_engine_version_1(self, mock_hvac):
        mock_client = mock.MagicMock()
        mock_hvac.Client.return_value = mock_client
        mock_client.secrets.kv.v1.read_secret.return_value = {
            "request_id": "182d0673-618c-9889-4cba-4e1f4cfe4b4b",
            "lease_id": "",
            "renewable": False,
            "lease_duration": 2764800,
            "data": {"value": "world"},
            "wrap_info": None,
            "warnings": None,
            "auth": None,
        }
        kwargs = {
            "variables_path": "variables",
            "mount_point": "airflow",
            "auth_type": "token",
            "url": "http://127.0.0.1:8200",
            "token": "s.7AU0I51yv1Q1lxOIg1F3ZRAS",
            "kv_engine_version": 1,
        }
        test_client = VaultBackend(**kwargs)
        returned_uri = test_client.get_variable("hello")
        mock_client.secrets.kv.v1.read_secret.assert_called_once_with(
            mount_point="airflow", path="variables/hello"
        )
        assert "world" == returned_uri
    @mock.patch.dict(