Support explain-SQL tests using external manifests

[plypaul]

This PR adds classes to support explain-SQL testing of external manifests. The main challenge for external manifests is described in this docstring:

class ExternalManifestSetupSource(ManifestSetupSource):
    """Provides setups from a directory containing external manifests (JSON-serialized).

    Example use case: "Here are a bunch of JSON-serialized manifests from customers. Check to see that a new release
    of MF doesn't break any of their saved queries."

    Since manifests from external sources may be authored using engine-specific SQL and rely on the existence of
    specific tables, several manifest transformations are required so that the manifest can be used with
    `DuckDbExplainTester`.

    In general, references to specific SQL tables and user-defined SQL must be replaced with similar dummy values.
    Since dummy values are used, the tests using the modified manifest won't be completely faithful, but can still
    capture some potential errors.

    Please see the associated transform rules for details.
    """