[MAINTENANCE] Send a Gx-Version header set to __version__ in requests to cloud

[wookasz]

Changes proposed in this pull request:

• Add Gx_Version: __version__ header when fetching data-context-configuration from cloud
• Add Gx_Version: __version__ header when performing any CRUD operations against cloud stores

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[Kilo59]

Change looks reasonable.

Tests should pass if you update this fixture to include the new expected headers.
https://github.com/great-expectations/great_expectations/blob/develop/tests/data_context/conftest.py#L605-L610

@pytest.fixture
def request_headers(ge_cloud_access_token) -> Dict[str, str]:
    return {
        "Content-Type": "application/vnd.api+json",
        "Authorization": f"Bearer {ge_cloud_access_token}",
        "Gx-Version": __version__,
    }